It seems that your kernel doesn't support USB audio driver.
Are you using current kernel source and rebuilt it?
Also we have to enable USB audio driver with menuconfig.
# e.g. enable USB sound adapter
# Device Drivers->Sound card support->Advanced Linux Sound Architecture->USB sound devices->USB Audio/MIDI driver[*] (Press Y), Save, Exit
We should be able to see these options in a properly configured kernel.
CONFIG_SND_USB=y
CONFIG_SND_USB_AUDIO=y
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