Well, same problem here, on the very same machine, after a fresh Debian Wheezy install.
Using the 3.2.0-1-powerpc kernel (is that the same on 12-04 ?)
lsmod shows the modules snd, snd_powermac, snd_timer, snd_pcm, snd_page_alloc and soundcore enabled, but alsa can't find any soundcard.
Running alsaloop gives :
Honestly, I'm still wondering what does the actual job of a soundcard on this machine.
ALSA lib pcm_hw.c:1401:(_snd_pcm_hw_open) Invalid value for card
playback hw:0,0 open error: No such file or directory
Loopback initialization failure.
It seems to be NEC hardware, and I've heard about a AOA driver (from gentoo people).
But I couldn't say more for now.
I'll keep looking around, keep in touch!