losing sound on my thinkpad

[Gary Moore/VENTANA <Gary_Moore/VENTANA.ITP@lgate.vmedia.com>]

I have a TP 755CD.  Recently I've begun having trouble with the sound.  
Everything works fine for the first 3-4 minutes...then the speakers simply stop 
working.  All goes quiet.  External speakers via the headphone jack continue to 
work.  The only way to get the speakers to resume making sounds is to turn the 
machine off and back on....rebooting won't work.  Any suggestions, similar 
problems???