You can write a little program to call the EMPEG_DSP_VOLADJ ioctl on /dev/dsp. Frank has already done this. If you wanted, you could write a little custom init script which first checks /proc/empeg_power, and then calls the ioctl with the appropriate numbers.
e.g.
Assuming you have Frank's program installed in /drive0/var
if grep "AC Power" < /proc/empeg_power > /dev/null ; then
/drive0/var/voladj 131072 0 65536 30 80
fi
Personally, I'm not going to change my patch to have this behaviour by default, because I (like tfabris) use my player at work, and it's good to have it just humming along at low volume. :)
Richard.