Music and ambient sound improvements

[pierow]

• The looping ambient sounds make a popping sound when the loop restarts. It's possibly a buffer underrun issue, so we can check the FMOD troubleshooting docs.

• FMOD isn't implemented on the linux client.

• The .wav ambient sounds on certain maps could use a volume remix, like the marine start on co_angst. This should be easy enough by editing the entity values in a bsp editor.

[romulasry]

Valve Makes All Steam Audio SDK Source Code Available Under Apache 2.0 License
https://github.com/ValveSoftware/steam-audio/releases