[Assuming you have a DS, not a DS Jr.] Does the DAC have a green dot in the USB input selection icon/button? If so what format are you playing and what format does the DAC display (e.g. 16/44.1k). If not, have you powered down the DAC AND the PC and then powered them up again?
I have sound over SP/Dif Toslink and also with a Sabre Matrix DAC I have sound over the USB interface.
With AP Linux, I think you should be showing a Card under “name” and not just “Audio USB”. Reboot with the DAC connected and turn on. Under DAC setup, click on List Cards, and I think it should show a card with PS audio listed.
Mine shows the following: Card 0: P20 [PS Audio USB Audio 2.0], device 0: USB Audio [USB Audio]