I'm having the same problem with music/sfx. As soon as I go into a race the master volume drops dramatically. Even with my laptop volume up all the way it's still too quiet. Interestingly if I go into the options the moment I click on a volume control the music goes back to normal. I return to the race, it drops again. Weird.
Did you install the game with the latest version of the installer for Powerslide we uploaded not too long ago? The update specifically addressed the music problem. If this persists even with the latest version, please write to our Support department about this and make sure to attach a DxDiag file to your email. Thank you :D
I did uninstall the game, and then redownloaded and installed the latest version, but nothing changed. Actually, I found out yesterday that after I installed nGlide 1.00, addressing a problem in a different game, that my audio volume levels were fixed! Maybe it had something to do with when the glide wrapper gets enabled when entering a race...