Hello.
My set up is Sondigo Inferno audio card - SPDIF - Behringer MS20 monitor.
I am going to replace SPDIF with RCA to try some of Dolby features on the sound card.
I searched internet and found hundreds of companies and products.
I am totally lost.
Could you guys suggest some good ones?
Thank you.

http://www.bluejeanscable.com/store/audio/index.htm

Not only an order of magnitude cheaper than almost any "HiFi" cables they are actually made by people who Have a Clue™.

Their page of correspondence with "Monster Cable" is particularly interesting, Monster threatened Blue Jeans cable recently over some very spurious patents on cable termination. What they didn't bet on was Blue Jeans being run by a retired lawyer.

http://www.bluejeanscable.com/legal/mcp/response041408.pdf

Is hilarious if you're into to that kind of thing.

I can second this recommendation.

I've purchased all my cables from them including a 25 foot run of component video cable for my projector. Waaaay cheaper than the brand name stuff and I'm totally happy with the quality.

Thank you for the suggestion.
But I can understand only few words on the page.
I am basically looking for a mini to RCA cable between my sound card and speaker.
Could you tell me if they have one?
Thank you.

P.S. Would there be a difference in sound quality between mini to mini and mini to RCA?

For any half-decent cable, there would be no difference.