because of an inferior codec that rejects my bandwidth
98.9% unlikely.
Steam/Source games use few codecs by default. My theory was just a possibility, and would NOT mute them in game.
It would just make them sound choppy during actual game play... it wouldn't mute them.
Now that you've given the fact they are actually being muted in Gmod, not just randoml silence..codecs wouldn't be at fault for that.
Also, besides what Megiddo is asking, are you running a listen server (server started in gmod?)
Or are you running a dedicated (srcds.exe) game?
If dedicated, can the players still hear each other, just not you?
If that's the case, it's not your server, it's your local copy...some lua or exploit muting it.