Musicians usually meet on the Music development forum. This is the best place to discuss new music and potential contributions.
Current information for musicians is found in the Project section
In-game music is in .ogg format, which is similar to the .mp3 format, only it's completely free. Sound effects are in .wav format, but can be in .ogg format -- this is crucial information for a campaign writer trying to reduce download size.
New music composed for Wesnoth should be orchestral with realistic sounding samples, and 3 to 6 minutes in length. For an idea of the direction the developers are looking to take Wesnoth's music in, check out Aleksi's and TimothyP's recent contributions.