Ubuntu on xM, howto turn off sound digital loopback?

Hi, all,

I installed the pre-build imange ubuntu-11.04-preinstalled-netbook-
armel+omap.img.gz on xM board.
Played with alsamixer for a while and seems got all sound working.

But I seems to bump into a problem: don't know how to turn off
"digital loop back" which will loop what's in input to output.

Only related settings I found are "Left/Right Digital Loopback" both
in Playback and Capture, they can not be muted in alsamixer, only can
turn them down to -18dB which is the lowest, seems that adds up to
-36dB of input end up in output channel.

But I am looking for a solution to completely get rid of input from
output, did I miss something there?



Anybody know which Driver file that I should look to check this issue?
is it some file under /kernel/Soc/Codec/ ?

Or did I missed somewhere to turn it off?