I have no problem playing MKV on default player in windows 10. It only happened on windows 10 mobile. I tried ax-lit player and it doesnt fix the problem. I will try to use best player tonight see if it work
most of the PC have hardware ability to decode Ac3, but most of the phone don't except phones like lumia 930, 830, etc. therefore you can play them on win10 on PC not win10 mobile. but if MKV's audio is ACC or mp3 codec, then the native player can play just fine. NOTE: AC3 or DTS codec is not built -in windows 10 since MS need to pay licences for them if they includes. On PC you can download codec packs easily, plus most of vedio cards have those hardware decodec. But on Phones, either if the hpone are DD plus, or you have to use software decode.
ax-lite is picky on AC3 too, but DTS seems fine. the developer of Best player is on this forum, you can request a promo code from him to get the full functional app, it is still working in progress though, so any issue you can just PM him directly.
There are also a UWP version of VLC player in progress now, should come out soon hopefully.