opkg update
opkg install usbutils
lsusb
...
Bus 001 Device 011: ID 0d8c:000c C-Media Electronics, Inc. Audio Adapter

Install USB sound card driver:

opkg update
opkg install kmod-sound-core 
opkg install kmod-usb-audio 
reboot -f

Confirm driver installed:

root@Arduino:/mnt/sda1# ls -al /dev/audio
crw-r--r--    1 root     root       14,   4 Jun 13 02:17 /dev/audio
root@Arduino:/mnt/sda1#  ls -al /dev/dsp
crw-r--r--    1 root     root       14,   3 Jun 13 02:17 /dev/dsp
root@Arduino:/mnt/sda1#  ls -al /dev/mixer
crw-r--r--    1 root     root       14,   0 Jun 13 02:17 /dev/mixer