I was gonna suggest that the song could be a sound file.
So if thats the case, just rename the uxx file into what it originally was (use cache.ini and look for the file name, copy and paste the cache name and renane that uxx file.)
Once thats done, it can normally be used in KF or opened in the SDK.
Music tab in the SDK is never used under any circumstance. It's a relic from the days when UE used mod music created with trackers.
Music should be ogg. This is an external binary so there's nothing to extract and should be in /Music/. If the author is using a sound as music (lol) it may be in mylevel or from a (huge) external uax. If this is the case keep in mind the sound quality would have suffered greatly so don't expect much from the exported wav.