Don't do this without headphones plugged in... you'll likely overload the audio subsystem with feedback.

From a shell prompt (i.e., inside `xterm` on the N770)
sudo gainroot
cd /sys/devices/platform/audio-i2c/
echo 1 > mic_enable
echo 1 > mic_loopback
echo 100 > gain_left
echo 100 > gain_right
echo 90 > volume_left
echo 90 > volume_right

Realize that as soon as the tlv320aic23 powers down (N770 goes 'dim') you'll need to re-enable the mic_loopback subsystem again by re-echoing '1' to it.

You might also want to dial the volume down after playing, 90 is almost max.

st_attenuation appears to apply filtering, but I haven't had enough time to play with it. Allowable values are at minimum {0, 1 .. 3}