Overview
As of version 3.30 it is possible to alter the music files utilised by the game without incurring the "modified" tag that prevents online functionality (this has been confirmed by Bernd in an AMA). Replacing the files works by making the game choose music files with the same name as the original. The steps to do so are outlined below:
Check the lists below OR use the XR Catalog Tool to see what names the tracks have: https://www.egosoft.com/download/x_rebirth/bonus_en.php
Put the desired .ogg files in the same folders with the same file names as tracks to be replaced. Example: X4 Foundations/music/Suspense/*
Launch game using -prefersinglefiles launch parameter
This only works if the music files are added in this way, any other form of modifying the audio files like creating a catalogue with the music files to be changed will incur a "modified" tag.
Files that can be Replaced
X4 (located in 04.cat):
X4 Foundations\music\BigBattle\fight_track_01.ogg
X4 Foundations\music\BigBattle\fight_track_02.ogg
X4 Foundations\music\BigBattle\fight_track_03.ogg
X4 Foundations\music\BigBattle\fight_track_04.ogg
X4 Foundations\music\BigBattle\fight_track_05.ogg
X4 Foundations\music\BigBattle\fight_track_06.ogg
X4 Foundations\music\Cluster\01. Alexei Zakharov - Beyond the Gates.ogg
X4 Foundations\music\Cluster\05. Alexei Zakharov - Where the Ravine Leads.ogg
X4 Foundations\music\Cluster\06. Alexei Zakharov - Oblivion's Siren Call.ogg
X4 Foundations\music\Cluster\10. Alexei Zakharov - An Era of Perfect Stillness.ogg
X4 Foundations\music\Cluster\13. Alexei Zakharov - On Frontier's Blazing Edge.ogg
X4 Foundations\music\Cluster\14. Alexei Zakharov - Turquoise Sea.ogg
X4 Foundations\music\Cluster\p1_amb_01.ogg
X4 Foundations\music\Cluster\p1_amb_01_voice.ogg
X4 Foundations\music\Cluster\p1_amb_02.ogg
X4 Foundations\music\Cluster\p1_amb_02_voice.ogg
X4 Foundations\music\Cluster\p1_amb_03.ogg
X4 Foundations\music\Cluster\p1_amb_04.ogg
X4 Foundations\music\Cluster\p1_amb_04_2015.ogg
X4 Foundations\music\Cluster\p1_amb_05.ogg
X4 Foundations\music\Cluster\p1_amb_07.ogg
X4 Foundations\music\Cluster\p1_amb_08.ogg
X4 Foundations\music\Highway\Highway_music_1_layer1_highsynth.ogg
X4 Foundations\music\Highway\Highway_music_1_layer2_percussion_a.ogg
X4 Foundations\music\Highway\Highway_music_1_layer3_percussion_b.ogg
X4 Foundations\music\Highway\Highway_music_1_layer4_drums.ogg
X4 Foundations\music\Highway\Highway_music_1_layer5_bass.ogg
X4 Foundations\music\Highway\Highway_music_1_layer6_brassstrings.ogg
X4 Foundations\music\Highway\Highway_music_1_layer7_hardsynth.ogg
X4 Foundations\music\Highway\Highway_music_1_layer8_snares.ogg
X4 Foundations\music\Highway\Highway_music_2_layer1_Ambient_Synths.ogg
X4 Foundations\music\Highway\Highway_music_2_layer2_Trance_Pad.ogg
X4 Foundations\music\Highway\Highway_music_2_layer3_Loop.ogg
X4 Foundations\music\Highway\Highway_music_2_layer4_Huge_Percussion.ogg
X4 Foundations\music\Highway\Highway_music_2_layer5_Bass.ogg
X4 Foundations\music\Highway\Highway_music_2_layer6_Beat.ogg
X4 Foundations\music\Highway\Highway_music_2_layer7_Staccato.ogg
X4 Foundations\music\Highway\Highway_music_2_layer8_Virus_Agressive.ogg
X4 Foundations\music\Highway\Highway_music_3_layer1_Pad.ogg
X4 Foundations\music\Highway\Highway_music_3_layer2_Melody_Virus.ogg
X4 Foundations\music\Highway\Highway_music_3_layer3_Loop.ogg
X4 Foundations\music\Highway\Highway_music_3_layer4_Massive_Percussions.ogg
X4 Foundations\music\Highway\Highway_music_3_layer5_Bass.ogg
X4 Foundations\music\Highway\Highway_music_3_layer6_Beat.ogg
X4 Foundations\music\Highway\Highway_music_3_layer7_Staccato.ogg
X4 Foundations\music\Highway\Highway_music_3_layer8_Agressive Virus.ogg
X4 Foundations\music\Highway\Highway_music_4_layer1_Pad.ogg
X4 Foundations\music\Highway\Highway_music_4_layer2_Virus_Melody.ogg
X4 Foundations\music\Highway\Highway_music_4_layer3_Massive_Percussion.ogg
X4 Foundations\music\Highway\Highway_music_4_layer4_Loop.ogg
X4 Foundations\music\Highway\Highway_music_4_layer5_Bass1.ogg
X4 Foundations\music\Highway\Highway_music_4_layer6_Beat.ogg
X4 Foundations\music\Highway\Highway_music_4_layer7_Bass2.ogg
X4 Foundations\music\Highway\Highway_music_4_layer8_Staccato.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_02.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_03.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_04.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_05.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_06.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_07.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_08.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - HOL_09.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Ambience 1.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Ambience 2.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Ambience 3.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Ambience 4.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Ambience 5.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Ambience 6.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 1.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 10.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 11.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 12.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 13.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 14.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 15.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 16.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 2.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 3.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 4.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 5.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 6.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 7.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 8.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Rebirth - Neutral 9.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Reunion - Drums Of The Goner.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Reunion - Taming Stars.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Reunion - Tranquil Light.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - Reunion - Yet to be Discovered.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - teladi_outpost_02.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - teladi_outpost_03.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - teladi_outpost_04.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - teladi_outpost_05.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - The Threat - Aurora.ogg
X4 Foundations\music\Legacy\Alexei Zakharov - The Threat - Orbital Surfer.ogg
X4 Foundations\music\Menu\MAIN_MENU_01.ogg
X4 Foundations\music\Menu\p1_amb_06.ogg
X4 Foundations\music\Special\Foundations.ogg
X4 Foundations\music\Special\HQ Reveal.ogg
X4 Foundations\music\Special\intergalactic_tea.ogg
X4 Foundations\music\Suspense\suspense_track_01.ogg
X4 Foundations\music\Suspense\suspense_track_02.ogg
X4 Foundations\music\Suspense\suspense_track_03.ogg
Split Vendetta (located in ext_02.cat):
X4 Foundations\extensions\ego_dlc_split\music\Cluster\02. Alexei Zakharov - Federation Lost.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\03. Alexei Zakharov - Heart of Acrimony.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\04. Alexei Zakharov - Guided By Unseen Hands.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\07. Alexei Zakharov - Rhy's Defiance.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\08. Alexei Zakharov - Dominion of Providence.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\09. Alexei Zakharov - Assembled for Eternity.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\11. Alexei Zakharov - The Stars Bleed Crimson.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\12. Alexei Zakharov - Echoes of Grandeur.ogg
X4 Foundations\extensions\ego_dlc_split\music\Cluster\15. Alexei Zakharov - Last Farewell.ogg
X4 Foundations\extensions\ego_dlc_split\music\Menu\16. Alexei Zakharov - Split Vendetta Main Menu (Bonus Track).ogg
X4 Foundations\extensions\ego_dlc_split\music\Special\scenario_combat_distress_call.ogg
X4 Foundations\extensions\ego_dlc_split\music\Special\scenario_combat_victory.ogg
Cradle of Humanity (located in ext_02.cat):
X4 Foundations\extensions\ego_dlc_terran\music\1. Alexei Zakharov - Cradle of Humanity.ogg.ogg
X4 Foundations\extensions\ego_dlc_terran\music\2. Alexei Zakharov - Wings of the Wind.ogg
X4 Foundations\extensions\ego_dlc_terran\music\3. Alexei Zakharov - The Traverse
X4 Foundations\extensions\ego_dlc_terran\music\4. Alexei Zakharov -Homesick.ogg
X4 Foundations\extensions\ego_dlc_terran\music\5. Alexei Zakharov -Pulse and Flow.ogg
X4 Foundations\extensions\ego_dlc_terran\music\6. Alexei Zakharov -Savage Soul.ogg
X4 Foundations\extensions\ego_dlc_terran\music\7. Alexei Zakharov - Citadel Rim.ogg
X4 Foundations\extensions\ego_dlc_terran\music\8. Alexei Zakharov - Rise of the Protectorate.oggForsaking the Void.ogg
X4 Foundations\extensions\ego_dlc_terran\music\9. Alexei Zakharov -Shining Currents.ogg
X4 Foundations\extensions\ego_dlc_terran\music\10. Alexei Zakharov -Machine Heart.ogg
X4 Foundations\extensions\ego_dlc_terran\music\11. Alexei Zakharov -Open Your eyes.ogg
X4 Foundations\extensions\ego_dlc_terran\music\12. Alexei Zakharov -Distant Dawn.ogg
X4 Foundations\extensions\ego_dlc_terran\music\13. Alexei Zakharov -Auspicious Creations - Main Menu.ogg
X4 Foundations\extensions\ego_dlc_terran\music\14. Alexei Zakharov - Insomnia.ogg
X4 Foundations\extensions\ego_dlc_terran\music\15. Alexei Zakharov -
This page is based on a reddit post created by u/apoptosist (tomchk on the egosoft forums) that has been edited for greater clarity and added Cradle of Humanity DLC music file names.