You can replace the original half-life mp3s with your own, by finding/making some mp3s and dropping them in the Valve\Media directory!

They can be a bit buggy tho--at least for me--, so here are some problems/fixes regarding custom target_cdaudio mp3s:

Problem:i'm trying to replace the mp3s with my own for target_cdaudio entity, but half-life overwrites them when i play the game, and plays the originals and not mine own.

fix: After you copy/rename--(half-life01, half-life02, etc) your own mp3 to the half-life media directory, make them read-only, and they should not be overwritten by the originals from the gcf file.
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Problem: I've renamed my own mp3 track to the proper filename--"half-life03.mp3" or whatev--, and i've made it a read-only file as well, but when i try to trigger it, STILL, some other song comes on?!

fix: fsr, at least for me, half-life renames your tracks in that, when you call on your file, say "half-life03", you would actually have to call up "half-life022" to make your custom song play.

The only way i know of to figure what HL has renamed your file to, is to create a test map of all 30 possible target_cdaudio sounds, with buttons, and go around clicking until you find your right file. THEN, you can enter the proper filename and your song will play.

Good news is it's static, once you figure out what it is, it should never change unless you delete it.
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Problem: You've done all of the above, but none of the buttons activate your custom track?!

fix: check the button targets and track numbers for your cdaudio "test file" and make sure everything is correct. (I have a test map and will upload it shortly, if anyone ever needs it)
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Problem: custom target_cdaudio: custom mp3s aren't playing, even though every other conceivable thing looks correct.

Fix: Try restarting steam. Sometimes, track changes will not execute unless you restart.
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^The above comment only applies to Steam HL. WON HL does not support mp3 files, only audio CDs.

I suppose you could place your own audio CD and it would still play. starts playing HL with Barry White soundtrack

The entity can only be used once. It kills itself automatically after it
starts playing.

Fun fact: Track 1 is unassigned because on the original HL CD track 1 is the data part of the CD. Track 1 is Half-Life.