I read here and in other sites that people use video converter to watch their mkv files because the stock video players (photo and gallery) cannot reproduce the audio track. This is true. But we just need an app to watch these videos.
MX Player gives us the possibility to watch mkv (hd and full hd video). No need to convert it or to install codecs.
When the mkv starts just tap on the note icon on the right corner and select "USE AUDIO S/W DECODER" (something like this, cause I have it in Italian) and you will see and listen your mkv video.
I'm not related to mx player and i don't want to advertise it, but this is the software i use, maybe there are other software that work too.
Hit thanks if I helped you