I dont see snd-usb-audio.ko. I might have been wrong, and may not have that included.
In that case the error message you received was because you were trying to install a module compiled for kernel version 3.3 (current openwrt trunk) on kernel version 3.0 (your currently installed openwrt). opkg allowes you to ignore the dependency and do a force install via option --force-depends. This is still not recommended but may work.
The clean solution is compile your own trx, using the exact options you want in menuconfig.
mpillaius, yes I did try plugging in my usb audio device (its the cheapy '3D Sound' $1.50 type shipped from China, which works on my WL-520Gu). It didn't light up and I didn't see anything in dmesg that it got detected properly (compared to dmesg on my WL-520gu). I'll double check tonight.