If the sound overdistorts or you hear high-pitched frequency pops, try getting whoever you hear having that problem to turn down their Amplification at Configure -> Settings -> Audio Input, turn off their Mic Boost, and/or turn off Automatic Gain Control (AGC), or a combination of all three.
If you are experiencing audio anomalies or glitches (robotic noise, audio cutouts, etc), many times, setting your Default Jitter Buffer and/or Output Delay to a higher value will fix it. Try playing around with the two settings. You can find them at Configure -> Settings -> Audio Output. It has been reported that 30 ms for Jitter Buffer and 30 ms for Output Delay works well for many people who experience these types of problems.
If the problems still persist, go to Configure -> Settings -> Audio Input and set everyone's Compression such that it shows "Speex" in the black text underneath the Compression box. Any setting that is <= 32.0kbit/s @ > 20ms audio per packet will be Speex.
If the problems persist, try setting your bandwidth lower at Configure -> Settings -> Audio Input -> Compression box.