I've never played this game before, I just started it and I'm in chapter 1, it doesn't have any sound in the game, I don't think I understood how to install it... Are the songs somewhere else?
First, you're on the mod thread, not the game thread. If you just downloaded the mod that's a problem (not sure how you're playing).
Second, if you're playing the raw game from the main thread with the mod and it's looking for the missing music files that could be the problem ... there are two versions of the mod, one for the "new" music and one for the original music, which needs additional files due to the musicopolips.
Basically, Drifty made agreements with music license distributor, populated game with said music, distributor changed it's own rules, Drifty launched game on Steam after finishing pt7, game was flagged as using music without approval, leaving Drifty out to dry.
He got as much of the original music back in as he could (working directly with artists) but couldn't get it all back in, so there are some holes.
The mod version that restores the original music needs the music files from the original version of the game, which is not available on the main game thread, but if you look at the notes in the download for the restoration version you should see links to find it (if they're still working, not sure what is anymore).
If you can't find them, then try reinstalling a fresh game and then use the "new music" version of the mod.