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Check System Preferences > Sound > Output or OPTION/Alt click the sound/volume icon in the Menu Bar and tell us, what is listed and checked as Output Device.

2012_01_04_pC1_OptionClickVOLUMEicon.png

Also check Applications / Utilities / System Profiler (Mac OS X 10.6 and earlier) / System Report (Mac OS X 10.7 and later)> Hardware > Audio to see, what devices are listed there.

As for external USB sound cards, what is your budget and what exact Mac OS X version do you use?
 
It says "No output devices found".

As for a USB sound card, I just need a good one to hold me over until the new iMacs are released.
 
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