audio channels

i have a 5.1 channel mkv video and when i enconde it in mp4, in the output file i get 4 channels only.

how do i fix it? i know the file itself is troubling. some people of mine tried to convert it in the defferent ways and had the same issue, but one got the 5.1 channel result with mp4box and took like thousand steps. but at least i know 6 channels are there...