Cut out voices, no bass, muffled sound, and a constant buzzing noise doesn't sound like tweeters or the bass speaker blown up and not working anymore...
If that would be the case then you would only hear either higher frequencies (eg voices) and not lower (so you cant here muffled sound in this case) or vice versa (but in that case you wont hear a constant buzzing noise)., if I get it right what you meant...
But it sounds exactly like what happened to my cheap speakers/soundcard after years of usage. At random times the sound just goes far away in the background, no real dynamics anymore and a lot of pink noise and some low pitched brum constantly overwelming the total sound...
This wasn't due speakers blowing up, loose contacts, interfearance or such stuff though. It was simply because of a worn out soundcard (eventually electronics can worn/burn out, and too much dust on the electronics maybe because the pc case was always open for years and not in a clean environment)
Maybe in such cases one should record the sound of the speakers via an external device so we can listen to it
/ 
