Copy source without transcoding

[ali2k1]

Hi everyone I'm using megui and I have lots of animes in mkv format. Firstly is there any way I can keep original video without transcoding and only converting audio from aac to ac3.
Input h.264 mkv aac audio

thanks .. I want to keep small size I've muxed them with mkv2vob but the files are like 300mb bigger than original.

Please help

[foxyshadis]

When you load the file in MeGUI, just run the audio part, and then drag the original video and the new audio into mkvmerge (http://www.bunkus.org/videotools/mkvtoolnix/downloads.html#windows). Deselect the AC3 audio, mux it to a new file, and you're done.