i had a similar problem. my mpd setup didn't want to play music anymore. no error message and so on. basically the same problems you ran into.

somehow my "cardnumber" seemed to be changed. calling aplay without parameters told me something like this:
aplay -l told me, that my audiocard's id is 1. so i changed the line
in my /etc/mpd.conf to
and everything was working nice again...

also alsamixer did not work, but calling it like "alsamixer -c 1" worked. this paramter forces alsamixer to open the mixers for card 1 and not card 0, which seems to be default if you have only one card installed.

i have no clue why this happened (i'm looking in your direction, udev), but i hope this information helps you finding the solution for your problems...

File Thingie is pretty cool...

did you follow the guide in the wiki about installing debian with ubifs on the internal flash? if you did, are you really sure, that you booted the kernel which you previously loaded into nand?

can you print your u-boot envs pls?