Music files MUST be .mp3 with 128 kbps or it will not work. Put them in the Music folder under Main UI, Team HQ or Workshop.
Texture files MUST be 24(?) bit .bmp that is a power of 2 by power of 2 large (32x32, 64x64, 128x128, 256x256, etc) or it will not work either. Decals can either be .bmp or .tga, but people use .tga because it allows for transparency.
Music does not effect sound, sounds are in the Sounds folder, of course.