USB powered Apple Pro Speakers don't work

view story

http://askubuntu.com – I just found my old Apple Pro speakers. Those are speakers attached to an external usb sound card with integrated amplifier. I'm using Ubuntu 12.04 LTS. When i plug-in the Speakers on any USB port Ubuntu recognizes them and displays them in the sound settings panel. the little amplifier also features a 3,5mm jack, when i plug in headphones they work. but i don't get any amplified sound out of the speakers as such. any suggestions? thanks a lot (HowTos)