Let's face it. We all know that SL's voice isn't totally stable, but there are a few things you can do to clear up a bad connection on the fly or make a dead mike work.
Dead mike - If your SL client is not recognizing your microphone (most common reason for a dead mike), go into (on viewer 2) ME - PREFERENCES - SOUND AND MEDIA TAB - INPUT DEVICES TAB then select your microphone. It should say something similar to (Microphone (Logitech Voicemaster USB) as one of the selections. Usually the selection labeled "default" is not the one you want. Only try it as a last resort.