An MKV container can contain pretty much anything and Roku supports a limited number of formats. You can find the list here:
How to use Roku Media Player to play your videos, music and photos | Official Roku Support
Using something like MediaInfo you can see what's inside those MKVs.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.
