This has been an ongoing problem with at least 2 of our users ever sense Opus was implemented. Our workaround is to force the server to use Celt though the overall sound is not near as good.
The server is Arch Linux 64 running the latest git snapshots and this problem still exists as of Feb 10 2013.
The clients are all Windows running current snapshots.
User 1: The most frequently affected has used multiple different MICs, headsets and has recently had to switch MOBO and CPU due to a recent MOBO failure. Sadly this didn't fix the problem. The problem also persists between windows 7 and 8. Next time he is in Linux I will test if he still has the problem. I'm attaching his dxdiag and an audio sample.
User 2: Affected far less often. Has nearly the same hardware as I. The only notable difference is he is running 3 AMD GPUs vs my one Nvidia GPU. He has recently switched from an M-Audio Producer USB Mic to a Logitech G930 and still the problem persists.
User 3: Me, not affected at all, other than the ear shattering pain... Only other notable differences from User 2 I can think of is I'm using a Killer 2100 Nic “Don't judge me I got it for free.” and i'm using a Blue Yeti MIC. I haven't had this problem in windows 7 8, Arch Linux 64 and FreeBSD 9.1.