After quite some time chasing various alternatives I have found a problem between the Linux kernel (usb core module) and the 730a MCP USB port. I have attached the nvidia-bug-report (with the USB device atttached, see lines 1438 and 2953) so I won't go into details of my system. In summary I'm running EVGA 730a motherboard with Ubuntu x64 v10.4
To get the "usb_set_interface failed" on line 2961 I simply opened gnome-volume-control and selected the USB audio device on the output tab.
Symptoms of the problem are failures of the Alsa sound mixer to control the output volume. The gnome applet continually reports...awaiting sound system response. The clue to the problem is revealed by dmesg. The kernel repeatedly reports usb_set_interface failed until the USB audio device is removed.
A little kernel checking shows the usb_set_interface call is in usbcore and being called by usbaudio. It described as follows "usb_set_interface — Makes a particular alternate setting be current"
BTW, I have concluded that it is the nForce MCP by using both Ubuntu 10.4 and Fedora 12 on my system with exactly the same result. I also have used the USB audio device completely successfully on my EeePC using Linux EasyPeasy (a Ubuntu variant with the same basic kernel version).
Any thoughts anyone has would be greatly appreciate by this oldcyberdude