Hello!

I have 2 Front speakers near my PC which I use for stereo listening (mp3s on my PC). I also have 2 speakers on the wall behind me. I want to watch movies(from PC) with projector on that wall behind me. I am using an optical connection from ESI Julia soundcard to Panasonic XR25 reciever. So basically I need to switch Front and Back speakers when watching movies. Is it possible through VLC(in movies) or Foobar (in music, instead) or other software/drivers?

Won't work.
SPDIF is just 2 PCM channels (=stereo) or a data stream (like AC3, DTS).

Well, I thought maybe it is possible to reverse speakers in data stream. Or for example, send the datastream with null fronts,sub and center, but put the music into the Back speakers through Foobar or other software/plugin. I am pretty sure it can be done, though I am not sure how.

To send multichannel audio to your receiver, you need it in DolbyDigital or DTS format. DVD movies already contain a DD or DTS track, which your software DVD player sends directly to S/PDIF-out without any alteration.

So, if you, for example, use a built-in DSP effect "Move stereo to rear channels" in foobar2000, you then need to encode the resulting signal into DolbyDigital or DTS. There are two competing technologies, DolbyDigital Live and DTS Connect, which allow real-time encoding, but as far as I know, they are not available as separate products.

The best solution would be to place your screen behind your monitor, and your projector behind you (if it's possible, of course).

ac3filter and/or ffdshow should be able to at least switch the speaker channels for you. Just use any directshow player (WMP, MPC, etc.) I think I read that the newest ac3filter has ac3 encoding for spdif, so that might do everything you need.