These are sounds which can be used within the webspace. Supported formats are dependent upon the multimedia capabilities of the underlying platform (e.g. DirectShow on Windows, gstreamer on Linux). Uncompressed WAV will work universally, but is not bandwidth friendly. MP2 or MP3 are compressed formats which seem to work universally and offer great compression.
http://janusvr.com/docs/build/assetstag/index.html#AssetSound
Music: https://www.bensound.com