First, make sure you have the latest drivers for your audio, video, webcam etc.
Second, I'd advise doing the audio and video wizard in WLM before starting each call if you are having problems.
Finally, I've had issues with different computers, headsets, etc that do indeed require reboot prior to use. I had one computer that the sound would just drop and only a reboot would fix it, and more recently my bluetooth headset won't allow for sound if I've used it, gone back to speakers, and then back to headset.
Can you give us info on what your particular computer is, what webcam or headset you are using, etc? There may be some info available based on the particulars.