This happened to us yesterday when playing a custom MP-mission, and this is the only time it has happened to me;
One of our players' sounds disappeared to the rest of us. He kept firing his rifle etc., he could hear it himself, but the rest of us didn't hear a thing even when standing one meter from him (no silencers or anything used). We did hear the bullet hitting the ground though, but no shot.
I am not sure if the bug came in the middle of the round or if it was from the beginning, coz we didn't do any shooting at the start of the round.
No idea how to reproduce. It seemed to just happen at random.