Changing Audio within a .mkv file

[pixiesoft]

I want to change AAC 6ch audio within a .mkv file to 5.1 or 2ch audio. How can this be achieved?

[Inspector.Gadget]

Take the original audio from the disc and do one of the following:

eac3to audio.ac3 audio.m4a
eac3to audio.dts audio.m4a -down2

etc.

[pixiesoft]

I click on the .exe file but the GUI doesn't come up? It just does a load of things very quickly in CMD Console and then thats it.

[Inspector.Gadget]

eac3to is a command line application. You would run it by opening a prompt and doing something like the following (read the documentation to figure out which parameters to pass):

To take the first audio stream from the first playlist on a Blu-ray disc in drive E and downmix it to a stereo file on your desktop (requires NeroAACEnc.exe from the Nero Digital AAC download to be placed in the eac3to folder)
C:\users\pixiesoft\downloads\eac3to\eac3to.exe E: 1) 3: C:\users\pixiesoft\desktop\audio.m4a -down2

To take the AC3 core of a TrueHD second audio stream from the second title of a Blu-ray disc you've already ripped to your hard drive without re-encoding or downmixing:
C:\users\pixiesoft\downloads\eac3to\eac3to.exe D:\BLU_RAY_DISC\BDMV\ 2) 4: C:\users\pixiesoft\desktop\audio.ac3 -core

eac3to is fairly straightforward: everything simple is done by choosing numbers of titles or streams and picking from several basic operations for a given audio stream.

[pixiesoft]

but this an .mkv file and not a blu ray disc

[Inspector.Gadget]

I'm telling you to start from the original Blu-ray disc rather than the file encoded from it so you're transcoding, if at all, from the highest quality audio source. Lossless -> lossy -> lossy or Lossy -> lossy -> lossy transcoding is suboptimal when compared to Lossless -> lossy or Lossy -> lossy transcoding.

Edit: I take it that this is a Blu-ray disc, since you posted in a Blu-ray section (if an incorrect one). If you don't have the disc, nobody can help you until you've cleared the file's origin with a moderator.

[pixiesoft]

I started to use RipBot and everything is ok now with no problems. A bit slow though!