No I haven't had that problem with my 9650. One thing you may want to watch out for is take care not to block the microphone port on the bottom left part of the phone. If your finger covers this your audio could be blocked.
I have the Bold 9650, and haven't had that problem at all. The microphone, as Spiral said, is on the bottom left. make sure your case or your hand is not covering it. Sometimes I'll try to hold the phone between my shoulder and chin and that's when my callers have difficulty hearing me. Better to hold the phone with my hands!!
Are you using a handsfree or bluetooth when this happens? What kind of case (if any) do you have on your Bold?
Yes, I have the same problem. It is random and very frustrating. It is not a manual problem like a blocked microphone. It's like it is on Mute. When it happens, the only way I can get it to work again is to pull the battery and wait for it to reboot. I called Verizon today and they reprogrammed the phone but I don't think that is going to fix the problem. They promised to call me back on Wednesday to see if I am still having the problem... We will see.