I was hoping for the actual MKV tag, probably something like "A_PCM/....". Something like mkvinfo (from mkvtoolnix) should tell you.
mkvmerge can also split files based on time code. Somethnig like this will split the first 30s
mkvmerge -o sample.mkv --split 30s source.mkv
Note that --split will split after all 30s, so it'll create alot of small chunks. But its the easiest way i know from the top of my head to get a sample out of it. Just stop it after ti created a chunk
