MCP51 AC97 on-board sound worked when I installed Ubuntu, then stopped!

[Adam Funk]

On Thursday I did a clean installation of Ubuntu 6.06.1 on a machine
with a Biostar motherboard.  The on-board sound has the following
description according to lshw:

     *-multimedia
          description: Multimedia audio controller
          product: MCP51 AC97 Audio Controller
          vendor: nVidia Corporation
          physical id: 10.2
          bus info: pci at 00:10.2
          version: a2
          width: 32 bits
          clock: 66MHz
          capabilities: bus_master cap_list
          configuration: driver=Intel ICH
          resources: ioport:dc00-dcff ioport:d800-d8ff
          iomemory:fe02c000-fe02cfff irq:209

The sound worked then.  At some point it stopped working, and now when
I run XMMS (for example) I get the "Couldn't open audio / Please check
that: Your soundcard is configured properly..." error.

I have the following sound-related kernel modules (which I think are correct):

$ lsmod  |sort |grep snd
snd                    60068  10 snd_intel8x0,snd_usb_audio,snd_rawmidi,snd_seq_device,snd_hwdep,
                                 snd_ac97_codec,snd_pcm_oss,snd_mixer_oss,snd_pcm,snd_timer
snd_ac97_bus            2688  1 snd_ac97_codec
snd_ac97_codec         99296  1 snd_intel8x0
snd_hwdep              10272  1 snd_usb_audio
snd_intel8x0           35804  0 
snd_mixer_oss          20800  1 snd_pcm_oss
snd_page_alloc         11592  2 snd_intel8x0,snd_pcm
snd_pcm                96772  4 snd_intel8x0,snd_usb_audio,snd_ac97_codec,snd_pcm_oss
snd_pcm_oss            56352  0 
snd_rawmidi            27552  1 snd_usb_lib
snd_seq_device          9548  1 snd_rawmidi
snd_timer              27204  1 snd_pcm
snd_usb_audio          84032  0 
snd_usb_lib            18432  1 snd_usb_audio
soundcore              11040  1 snd
usbcore               138948  6 snd_usb_audio,snd_usb_lib,pwc,ehci_hcd,ohci_hcd


I'd appreciate any suggestions for fixing or debugging this.

-- 
Thanks,
Adam