iMac sound problem

[rediffusion]

My iMac's sound keeps cutting out when playing music. When I try to increase the volume I get this:

After a short time the music comes back and I can increase/decrease the volume as normal. Any ides what's causing this?

thanks,
R

 

[Hellhammer]

What speakers are you using?

 

[rediffusion]

What speakers are you using?

I'm using Cambridge - SoundWorks speakers connected to the headphone jack.

 

[stridemat]

Mine does that when the headphone jack is not in correctly.