Don't know if you checked Google's archive of alt.video.laserdisc from around 2003, but Karyudo's success in doing this was discussed around this time.

Here's one discussion:

Donald Haas said:

What Audio card allows the saving AC-3 right from the TOSLink datastream without decoding?

Karyudo said:

Ah, now there's a question! This is the hardest part. Because many
cards are TOSLink/coax capable, but then mangle the bitstream. Even
some cards with the same chipset do different things: one will be
fine, and another will butcher. You're gonna have to try some, I
guess. I've got an ISA card that works fine, but it's ISA. Some cards
-- even quite inexpensive ones -- with the CMI 83xx (<-- forget the
exact number) chipset are OK; others don't work.

This is one arena where throwing more money at the problem isn't the
fastest way to a solution. A cheap-o CMI-based card could well
out-perform (in this aspect, anyway) the most expensive Creative
Audigy something-or-other.

Best of luck -- and please post your results (good or bad), because
that's what everyone's looking for!

D. Carroll said

Sound cards that have codecs (coder/decoder) that conform to the AC97
spec will resample all audio to 48kHz.

Most "consumer" (read cheap) soundcards conform to the AC97 spec.
AFAIK, the only cheap soundcard that doesn't conform to the AC97 spec is
those based on the Cmedia 8738 chipset (thanks to its built in
proprietary codec).

However, "Prosumer" (read expensive) cards based on VIA's (formerly
ICEnsemble's) Envy24 don't resample audio. M-Audio makes some nice
cards. Too bad their relatively cheap new consumer card based on the
Envy24, the Revolution, doesn't have any digital inputs.

I heard that Microsoft is working on an update to AC97 but knowing
Microsoft there will undoubtedly be some catch.

Karyudo has not been on here in years, but I know that Darth Editous and adywan have also managed to do this.