Copy/pasted from a post in the L3DEdit topic (as this patch used to be part of L3DEdit):
The music patch does not fix anything with sound working or not working.
What it fixes, is that in the vanilla version of L3D, there's a discrepancy between CD audio and other audio. As you might know, there's two tracks for each style on the CD audio (I'll call these "Standard" and "Alternate"); while the soundcard / MIDI audio has three for each track (equivalents to Standard and Alternate, plus a third one - which I'll call "Remix" - which is always a remix of Standard).
Each level can select between Standard, Alternate and Remix. Obviously, this setting is based on soundcard / MIDI audio. Since CD audio doesn't have Remix, you'd logically expect that any level that plays Remix, would instead get Standard under CD audio; while Standard and Alternate would work the same way they always do. However - what actually happens, is that while Standard works fine, the CD audio will play Alternate if the level's set to "Remix", and Standard if the level is set to "Alternate".
The music patch fixes this, so that the musics are consistent between the CD audio and the soundcard / MIDI audio. That's all it does - specifically, it does this by swapping the 2nd and 3rd music file for every style. You can then choose, either globally or per-style, whether you want to treat the CD audio track selection or the soundcard audio selection as the "correct" one (which will then patch the levels appropriately - when keeping the CD music, or on levels that use Standard, no changes are needed; otherwise, it will swap between the two remaining options). You can customize this however you like, but I've set the default to be keep soundcard / MIDI's choices for Space levels, and CD audio's for all others (when you see exactly which Space levels get the alternate music with this setup, you'll see exactly why I made that the default).
If sound / music is not working, you need to run L3D's "setup.exe" and configure sound hardware. With DOSBox's default config, you can use either Soundblaster or General Midi. With some additional setup, you can also use Gravis UltraSound. AWE32 is not supported in DOSBox - although you can use Virtual Midi Synth, together with an AWE32 soundfont and the "General Midi" music option, to get very close to AWE32 music (but you just have to settle for Soundblaster or Gravis UltraSound for effects) - the one catch being that it glitches out hard towards the end of the Standard track for the Computer style (sound glitches only, but they're REALLY annoying). The need to run setup to configure your sound hardware is nothing special to DOSBox; the same applies to running L3D PC version on real hardware.