When I switch to other channel, sometimes sound gets corrupted or there is no sound at all. If I reswitch that channel again or switch to some other channel - sound appears... ProgDVB 5.15.5, cyberlink video decoder (PDVD8), cyberlink audio decoder (PDVD8), VMR9. Tried with other decoders like NVidia but still the same.

Try to find out, from and to what audio modes you are trying to switch, exactly.

I've made a test by switching channels and this is what I was able to find out:
Switching FROM -> TO modes (TO means that audio becomes corrupted or disappears):

mpg2/384/48000/2CH -> mpg2/96/48000/1CH
mpg2/192/48000/1CH -> mpg2/192/48000/2CH
mpg2/64/48000/1CH -> mpg2/192/48000/2CH
mpg2/192/48000/2CH -> mpg2/192/48000/2CH (same mode!)

ALSO I've found out that when REswitching the same channel sound disappears (or gets corrupted) too, it means from one mode to the same. But I noticed that it happens only reswitching channels with this mode - mpg2/192/48000/2CH.
I'm not 100% percent sure, maybe there are more modes that gets corrupted, if this is not enough, I can do the test for a longer duration.

P.S. Juergen, could you please tell me why does it take so long to switch to other channel? I mean 5 to 6 seconds. Somehow set-top-boxes do this job in 1 to 2 seconds. I've noticed one strange thing: when I switch to a channel that is in different frequency it takes ~5-6secs (some HD channels ~10-15secs), but when I switch among channels with same frequencies usually it takes ~1sec.

Does my pc participate in channel switching process? Or is my dvbc card so slow?

First reason for switching delay normally would be, having to wait for a new GOP to start with an I frame. Unavoidable, but should be below a second. Same as on an STB. May somewhat differ on HD, possibly also compression related.

Then there are buffer settings on the application, specially minimum buffer size. A timeshift controls issue. Too high means additional waiting.

Also possible, a driver slows things up, delaying after channel change, to give the tuner a chance to stabilize. Nothing to be done except of a possible driver update. This would be typical if switching to another transponder needs more time.

And the application may as well cause extra delays for many reasons. Compare with another one.