Need a converter that will only con’ the audio & copy vi

i have 1080p .mkv file with dts audio, want to change the format to ac3, and the video only copied,saving hours of proccessing.
xilsoft have not copy option and avidemux gave me errors.
thanks
Answer #1
You could try MKV2AC3 or PopCorn..
http://www.videohelp.com/tools/MKV2AC3
Answer #2
thanks again