Okay, this has happened twice to me already so I am pretty sure it is a glitch/bug. Midway through the skirmish (actually about 5 to 10 minutes in it), the sound effects such as the battle sounds and voices disappeared, leaving only the music playing. Then after the skirmish ends, sound effects continue to be missing in the landscape. I had to exit and restart the game to get the sounds back (logging and relogging didn't work). Anyone else having this problem or is it just me?
In any case, I have already submitted a bug report so they could look into it.