I see... Strange, pitivi plays this mkv without any issues. So I converted audio-track into 2-channels ogg with vorbis codec by pitivi and when inserted that converted audio-track into kdenlive it plays without noise.

pitivi uses a different backend (gstreamer), kdenlive uses mlt as backend, this could lead many differences.

I will ask the mlt dev why ac3 6ch plays fine and why aac 6ch not.

... But, I've found an interesting script here http://forum.xbmc.org/showthread.php?t=62652 which converts this aac-track from any mkv into 6-channels ac3-track and this ac3-track plays without noise in kdenlive.

Thank's

Mlt dev here. libavcodec lets you request a number of channels, and by default Mlt asks for 2. The AC-3 decoder is able to fulfill that request by doing a downmix, but not the AAC decoder. Mlt can handle >2 channels of audio, but there is a bug at the moment when the decoder does not fulfill the request and returns all channels. Mlt still tells its downstream components there are two channels when in fact there is not, there are 6.

If I simply fix that bug and tell the downstream there are 6 channels, there is still a problem. The output component (SDL, avformat) will get 6 when it only asked for 2, and it might not how to handle that. Sure, I could make each output component use only the first N channels it requested, but in the case of the sample file provided here you would lose all of the dialog. What I need to add is some sort of downmixer filter that will reconcile the actual against the requested number of channels - like we already have for sample rate.