looking for app to rip sound from video

What is the original file format of your video with audio?

 

You can do an Export Audio Only option in Quicktime.

 

As some refer to MKV as a wrapper, it means whatever the original format was, is "inside" the MKV wrapper. Thus, an mp4 can be converted to MKV and something like an m2ts file could be as well. The mp4 might have simple audio while the m2ts have PCM or DTS-Master etc. As you can see, these audio portions are quite different.

Given that MKV files may have been conversions from all sorts of files, if you don't know what is inside, then you may want to examine the MKV files with various software. I know that on the Windows side, there are all sorts of free software to do this. (This is where a minimal virtual Windows is ideal). Perhaps there are some counterparts on the OSX side but at present, not aware of "free" ones.

 

OP:

You didn't tell us which -file format- of audio you are trying to achieve.
Wav? AIFF? mp3? Something else?

If all you need is mp3, then I suggest you try the free app "ALL2MP3".
It will "batch convert" to high-quality mp3 format...