I have a Andersson R2 AV receiver which supports PCM 96 24 2-channel digital signal. I have connected it to my PC with a coaxial cable to the spdif on my Realtek soundcard.
How can I send PCM 96khz 24bit 2-channel digital audio signal from my PC to the receiver? I have the possibility to change the soundcard signal to 96khz, but this makes normal 44,1khz soud really bad. What i really would like is to use some kind of passthrough function in a player like foobar2000. Is this possible?

AC3 and ffdshow DirectShow filters have some passthrough options in their configuration IIRC, try installing them and playing the files via media player (classic) or just any other media player which uses DirectShow. They come as a part of K-lite Codec Pack.

Tried that and it does not do it for me. Any other suggestions?

That shouldn't happen... This might be a silly question, but have you checked for driver updates? (This is probably a driver problem.)

Realtek seems to have a poor reputation, but I don't know if it's deserved... It might just be that since they seem to make most of the soundchips, they get most of the complaints... ???

Well, you have to make sure these filters are actually used in playback of PCM audio (this is not automatic). You may check if AC3 filter is used for playback in media player classic by right clicking on the player and choosing "filters" submenu. If you don't see the desired filter there, you may have to tweak the codecs settings of your system (like boosting priority/merit of the desired filter).
It is not enough to just install the codec pack since the default "decoder" for PCM audio will not be the AC3 or ffdshow codec (quite logically).

To the best of my knowledge, even though PC soundcards (including onboard) have optical out with enough bandwith, they still all use the S/PDIF protocol which is basically designed for raw cd audio or dolby digital / dts. I don't think they can transfer hi-res or multichannel audio uncompressed, but I'm not sure.

I managed to force AC3 codec to handle PCM audio. The sad story is that there is no SPDIF passthrough option for PCM data (1).
But there sure is an option to send audio data uncompressed over SPDIF as pointed out in (2).
There is also an option to restrict SPDIF sample rate, which means that if it is unchecked, it will allow sampling rates over 48kHz to be sent over SPDIF.
Also, there is a warning when you try to check (2) saying "This option is dangerous! It may produce a very loud noise...".

M-Audio Revolution7.1 sound card supports 24bit/96khz digital output:)