-I was wrong on something: the extension should not be written in the filename (according to the Lua API).
-We need more info to find the problem. This could even be an engine bug, but that's unlikely to be the case, so I want to be sure before we open an issue. Questions:
1) Which version of Solarus are you using?
2) Is that file added as a quest file in the Quest Editor?
3) Which is the extension of the file? (Some audio extensions are not supported by Solarus)
4) Is that music being played when attached to a map in the map editor?
It would also help a lot if you can temporarily share a link to your folder, so that we can help you solve this problem.