Missing Audio Output devices?!

Hi all
For some inexplicable reason my internal and external speakers are no longer an option in 'sound' in system prefs. When I click on the output tab the only option I get is external headphones (which works fine). My system profiler has the following:
Built In Sound Card:

CODEC: Texas Instruments TAS3004
Sample Rate: 44.1 KHz
Number of Inputs: 2
Number of Outputs: 3
Devices:
Input:
Type: Line Level Input
Input:
Type: Internal Microphone
Output:
Type: Headphones
Output:
Type: Internal Speaker
Output:
Type: External Speaker
But no matter what I've tried I cannot get the speakers to work, physical connections seem fine and not even the internal speaker works when all ext devices are unplugged.
If anyone has any suggestions I would be most grateful.
BTW, there was no big software/hardware change that caused this. One day they worked, the next they didn't!
Many thanks