I have a file in .mkv format

I want to run it through Format Factory, leaving the video unchanged

just changing the Audio from DTS to AAC

The Audio settings are easy, but what to specify for the Video to leave it unchanged?

Thanks in advance

Just change audio encode settings and leave the video settings as they are. Click OK and then start.

In Format Factory set the Video Stream (Video Encode) option to "copy."

I would suggest getting a copy of MKVToolNix (a set of mkv tools) - MKVMerge GUI is very useful for working with MKV files. You can add audio tracks, subtitles to mkv videos, or put avi and subtitles into an mkv container. You can also remove unwanted audio and subtitles from mkv files.

Also get a copy of Avidemux, which will let you convert the audio format.

Both programs are FREE and very useful programs.

Thanks for the suggestions, but still no progress.

The Format Factory settings look good, but the programme does not cope,

hanging at 89% every time

I downloaded the Avidemux as suggested but neither the 64bit or 32bit versions will run under Win 7

The programme installs OK, but nothing happens when I click to run the programme.

Very odd indeed.

I have checked with the task manager CTRL+ALT+DELETE but no process is started

Resorting to Google for other alternatives I came up with

Popcorn MKV Audioconverter and I am running the file through there.

Will report back later.

Yes that did the job and quite quickly too.