-this happened to me on 2 different servers with 2 different gamemodes (one of them was samatra's king of the hill, the other was a zeus game)
-in king of the hill, everyone noticed the left channel problem. It happened to most of the guys at the same time, and apart from that, there were individual mutes too.
-in the zeus game, that only happened to me whenever i drove a car and switched to first person view.