The newer Apple USB audio drivers (at least, as of Lion) do not turn on the speakers, but there is an easy fix described at the end of this thread on Apple support. (Ignore the earlier posts about modifying the circuit board, it’s not required.)
In case the thread goes away, you download the old version of AppleUSBAudio.kext thoughtfully posted online by a forum poster here.
Then find and download Kext Wizard (I used v3.7.10) and use it to Install the older AppleUSBAudio.kext file.
As noted in the thread, it can sometimes be a little squirrelly when changing volume using the volume keys on a keyboard, sometimes a quick unplug/plugin is required. But the sound is sweet and the look is still classic Apple-cool.