Musicians usually meet on the Music development forum. This is the best place to discuss new music and potential contributions, as well as to hear what is currently being worked on.
Wesnoth's music is stored in the Ogg Vorbis format. This is similar to the MP3 format, only it is completely free. The current music can be found in the "music" subdirectory of Wesnoth (see EditingWesnoth for details on Wesnoth's directory structure).
Music composed for Wesnoth should be orchestral with realistic sounding samples, and 3 to 6 minutes in length. Most of the pieces in the "Core Music" section of Available Music are good examples of this.
Where Should I Start?
The first thing any potential music contributor should do is read the Music Contribution FAQ. It outlines what Wesnoth's music development team is looking for in terms of contributions, and who is likely to be able to contribute work that will be accepted into the core Wesnoth distribution.
If after reading the FAQ you believe you can contribute something to the project, introduce yourself on the Music development forum. You will most likely be asked for a sample of your previous work, so if you can provide a link to one in your post it would be very helpful.