.wav for sounds, .ogg for music. Both have to be mono.
For music:
Put it in the music folder inside of your add-ons folder.
For sounds:
Put it in a folder, with the name style of Category_Name. Put said folder inside of your add-ons folder, and create your description.txt and your server.cs If you do not know how to create these two files then look at other similar add-ons.