sound has been broken ever since the beta times , i often dont hear teleports or item pickups.
Theres a few spots in maps where enemy sound goes completely mute and its easy to reproduce the effect.

check the value of com_soundMegs, if its below 16 or so try increasing to 32 or 64 and see if that changes anything. The broken sound problem especially in quad damage case is an age old problem, mostly happens because lot more shit goes around quad in a game and the sound buffer gets overcrowded and often i have seen sounds dont pop in right order or some of them get dropped.