I had this happen as well, but only once or twice.  Returning to the overworld map fixed the issue.

I've had this too after playing for like 150+ turns straight. 

I get sound issues as well.  During my last tactical battle against the Troll and Slag in the campaign there were no battle sounds just ambient bird singing in the background (which even after quitting was still playing for several seconds).  The Troll and Slag died in eerie silence, while in previous battles I had battle sounds thhroughout.  I could not get the birds to stop their singing, hence the reason I had to quit.

Also I barely ever get any particle effects for spells in combat and also the arrows fired by archers are also invisible.  The camera theat follows the shots is also VERY annoying, it does not snap back to the firing party and if you fire again and the unit is off screen the camera does a very weird flyby that actually made me dizzy in one instance with all the tilting back and forth it subjected the viewer to.  An option to turn the follow camera off or just have it snap back to the firing party after its hit would be very much appreciated.

/signed

Yup I get loss of sound/music and sound effects sometimes one or both and it seems totally random and sometimes it comes back after awhile.  Luckily for me, I do not require sound to enjoy the logic and game mechanics, but it's a minor bug that is still a bug.

I've noticed that the sound seems to drop about the time the game starts getting slugish.  Seems to me since the 1.06 patch that the game gets really sluggish in tactical combat a lot earlier too.  I'm having to Right click 4 or 5 times before my attacks, moves or shots take effect.