Volume mutes automatically

The output volume on my Macbook Pro keeps switching to mute automatically. I have gone into Sound Control in System prefs and unchecked Mute but the sound just goes back to Mute. Tried Audio Mini setup -same thing. There is no difference whether I use headphones or not.

I have tried repairing disk permissions. Tried ramping the sound up using the F12 keyThe only way I can get audio is to permanently hold the volume slider in position. The only thing I can think of is that OS 10.5 is clashing with my SnapzPro software (which captures video and audio) or that maybe a preferences file has been corrupted? I have no external hardware connected. Please help. It is very frustrating.