Suddenly no audio output via external speakers

[Supernov]

Suddenly and out of the blue, no sound from the external speakers. When switching to internal speakers, sound comes out but volume is stuck and no doesn't respond to the slider.
Tried resetting PRAM but the problem remains the same.
Tried switching the plug in-out several times, same case.

iMac 27" , OS X 10.6.8
Please help
TIA

 

[chscag]

Try an SMC reset: Shut the iMac completely down. Remove the AC plug from the outlet and let the iMac sit unpowered for about 15 minutes. Plug it back in and reboot. Is the sound normal again?

 

[Supernov]

Dear chscag. It worked!!!! One million thanks are not enough. You saved me a trip to the store...
Thank you!