I noticed that my garrysmod folder didn't include the sound folder. Well, the same is too other people too. Luckily, I found a fix to this. First, go to C:(DEPENDS ON OS)\steam\steamapps\GarrysMod\garrysmod. Then, make a folder called sound. Then, go inside that folder and make another folder called gmmp. Then, you're basically done. Just move the mp3 files you want to listen to inside of Garry's Mod and they should show up in your list.