Ok, I did the Slackware64 14.0 + Slackware32 14.0 test and everything worked fine.
I've also tested the other machine (Slackware32 14.0) with Windows and the same horrible sound could be heard.
Now, all those tests were performed on my Murmur server running on Slackware64 13.37.
Next step was to use my freshly installed Slackware32 14.0 and add Murmur to it, so I could test if maybe it's a server issue. It's not. Same thing happened.
So maybe it's Murmur on Slackware that's acting weird? I've installed Murmur on Ubuntu 12.04. No difference.
After concluding all those tests, it seems that it's not a hardware issue (used 2 slackware machines), and the only situation where I get the disturbing noise is when Slackware wants to speak with Windows (both on my Vista and my brother's Windows 7).
Very weird issue.
This solves the problem, thanks bw42
However, it's only partially solved. When I followed that tutorial, I could be heard on a Windows computer, but I could hear only the metalic/robotic sound. When Mumble on Windows follows the same tutorial, then I can hear the voice.
I use Mumble mainly to communicate with my brother, so it's not a big deal, since I could easily make him change his Mumble configuration, but what about other people? If my gaming guild used Mumble instead of TeamSpeak, I'd have to convince 50+ people to alter their settings.
Besides, why is Ubuntu, with the same Mumble settings, able to speak to Windows and Slackware not? I'm curious why kingbeowulf
's setup works just fine. Can it be a firewall on a router issue? But it doesn't block anything in my LAN.
Argh! I have no idea
P.S. I've marked it as solved, although it's not fully solved, so if anyone has other ideas...