Hi, this is my second post, and i hope it can contribute to the comunitiy .
This night i was "surfing" in apple's website and in applications i have found an app called AudioSwitcher 1.2.1 it could be found here: Audio Switcher 1.2.1 and as i have pluged-in a pair of usb speakers, now i can switch between Digital Audio Out, Line Out and USB Audio. Also It can Select From Mic In to Line In. I have only tested Line out and USB Audio. And i am using, if i am not mistaken Azalia Audio from Insanely Mac: Audio Azalia Codec.

Before using this app my sound was choppy and some kind of rubbish, now is perfect!! with a Sennheiser headphones, and with my logitech USB Speakers.

Hope to help.

Bye.