Most of the information here is for Warzone 3.1 and above.Overview
Useful links for working with the .ogg audio file format used by Warzone...File converters
There are a number of free tools for converting popular audio formats (like MP3 or WAV) in to the Ogg Vorbis format that's used by Warzone - you can find a big list on the Ogg Vorbis website. 
Free audio samples
There's loads of open source audio libraries on the interwebs, some are listed below. Always check the licence and make sure it's GPLv2 or other certified open source license before using it in a mod.
- Autistic Lucario – some audio samples from Arreon
Got more? Please edit this page or post a comment below.