One simple thing to check if you're getting poor audio quality (as apposed to no audio at all) is to make sure you don't have some background service tying up all of your internet bandwidth. I had a customer who's call quality was perfect for several months, then one week it became awful. After testing, it turned out that one of the kids had installed a torrent downloader/server that was wide open with no throttling. It tied up almost all the bandwidth, so calls were dropping packets and sounded terrible. We shut off the torrent client and all was well again.
Viruses can have the same effect, so make sure your systems are clean.
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