Having audio echo problems, anyone know a possible cause?

I'm using a Lenovo laptop with dolby digital audio, sometimes the audio goes into random echos and the silence button does not respond, a restart fixes it but I've been wondering what could be causing it.

Microphone is on, picking up sound from laptop's speakers, pumping sound back around through amps and into speakers again and again.
Turn microphone off or down in sensitivity. Or use headphones instead of speakers.
Guessing from no model number of "Lenovo laptop with dolby digital audio", no hint if you use speakers or headset, no operating system version.