I am running SUSE 9.1 on ASUS 7N8X-E Deluxe motherboard with AC97 codec. All sound applications (XMMS, Mplayer) have voice removal effect and I do not know how to disable it. XMMS does not use such plugin. I can't listen movies and listen music... just karaoke

lspci:
0000:00:05.0 Multimedia audio controller: nVidia Corporation nForce MultiMedia audio [Via VT82C686B] (rev a2)
0000:00:06.0 Multimedia audio controller: nVidia Corporation nForce2 AC97 Audio Controler (MCP) (rev a1)

I even do not know after what this effect appeared.

How do you have your sound connected? SPDIF? Or is it a stereo mini-plug?

I suspect it's the latter, but that your computer is producing six channel output. Try plugging the stereo mini-plug into the other plugs to see if you can hear the voices. Basically, the six channels of output are:

1. Center speaker (voices in center, other sounds/music)
2. Subwoofer (low pitch sounds and sound effects)
3. Left front speaker (mostly music and background sound)
4. Right front speaker (mostly music and background sound)
5. Left rear speaker (surround sound effects and some music)
6. Right rear speaker (surround sound effects and some music)

It may be that you're only plugged into the left front and right front speaker outputs.