No sound, for no apparent reason

Since I upgraded from Debian's (Sarge's) stock 2.6.8 SMP kernel to the corresponding 2.6.16 linux-image from backports.org a couple of weeks ago, I usually get no sound from the speakers (Harman/Kardon HK195) connected to the AD1985 AC '97 audio hardware on my Intel D865GBF motherboard.

Short version:

The audio hardware is detected. The usual modules load. I see no error messages whatsoever. All of my usual audio software runs normally, as if only the speakers were dead, or as if a mute button had been pressed; but the speakers are fine, and I don't see anything muted. Is this issue a known bug in the 2.6.16 kernel? If so, has it been fixed? If not, how might it be diagnosed?

Long version:

I say the audio "usually" doesn't work with this kernel because it did suddenly start working for a few hours, just once, and then it fell silent again. I couldn't guess why; I hadn't knowingly done anything that day to make it either work or stop working, and no unusual messages spewed.

The whole system seems to assume that the sound is working, but only the beep command for the system speaker gets me anything audible.

Running lspci shows me the audio controller. Running lsmod seems to list all of the right sound modules (I didn't knowingly add or remove any during the upgrade), and none are said to be unused. The file /dev/dsp exists, and "sound" and "alsa" show up when I run cat /proc/devices.

No error messages spew when Linux boots up and the audio drivers load, either, or when I run play, saydate, jacklaunch, timidity, or any other command that used to make noise. I never see "No such device" errors. XMMS doesn't tell me to check the configuration; it plays my files as if I could hear them. If I run artsd, esd, or jackd, the daemon always starts up normally and has no complaints.

A message says on startup that aumix is going to leave things alone because my configuration file was found, and the mixers that I (knowingly) run always confirm later that nothing was muted.

These (non-)symptoms would seem to suggest that the speakers are bad, but these speakers still work fine when I boot Windoze on the same machine. The speakers also make the expected crackling noises when I unplug them from the jack and plug them back in while Linux is running. I plugged in another pair of old speakers (Altec Lansing ACS5) that work on another machine (and used to work on this machine, too, with the 2.6.8 kernel) and still heard nothing.

Then I tried plugging these speakers into an Intel BI440ZX motherboard. On that machine, the es1371 driver had worked until today, with exactly the same kernel image installed a few days ago (because it was the only 2.6 kernel I already had that was new enough to have a working via-rhine module to fetch another kernel :). But now no sound comes out of that machine, either. I see the same apparent lack of any other symptoms, and the same speakers still work on that older Intel box, too, when I boot a Damn Small Linux CD with a 2.4 kernel.

So, the culprit seems most likely to be hiding somewhere in that 2.6.16 kernel or in one of its modules. I searched for known bugs and didn't see precisely this same problem mentioned anywhere, but I'm no expert in choosing the best sites or keywords for this kind of information.

What might the problem be, or what else can I do to diagnose it? I could probably put a backported 2.6.17 kernel on both machines without too much trouble if that's likely to help, but it would be inconvenient right now if a configuration problem is more likely than a bug.

[EDIT: I waited awhile and then linked to this post from debian because everyone here seemed to be stumped as thoroughly as I am.]