Might be a result of new sound code.
A lemming that is to the left of the current on-screen area, if triggering a sound, should have the sound somewhat panned to the left. Instead, it's panning to the right.
Haven't investigated or even reproduced this yet; was mentioned on IRC and I'm creating a topic so I don't forget.
Wasn't the wrong side, but rather completely wrong positioning altogether. This is now fixed for next update.