From The Battle for Wesnoth Wiki
the [music-list] tag
This tag is a subtag of [scenario] which is used to describe a music track to play. You can repeat this tag as many times as you like: one will be selected at random during scenario play.
Tags describe the music:
- file specifies the music file, relative to '"music/"'. This is compulsory.
- ms_before (optional) specifies how many milliseconds to delay before playing this track. Currently this does not apply when the scenario first starts.
- ms_after (optional) specifies how many milliseconds to delay after playing this track.