I've noticed more than a few times that pressing 'x' and clicking on the map (sometimes) produces no sound.
Jammy commented in IRC that he also has noticed this.
I tried reproducing in a game with filux today, but failed.
If anyone else notices this on the git version, please mention it, and try to note what you were doing at the time.
I can't say exactly what I was doing at the time, but will pay more attention in the future.