Convert them with ffmpeg or something.
What can I do to make them work without converting them?
You have to convert them.
I know a step by step guide
that aims to convert M2V files to more popular formats like MP4, AVI, etc.
It also introduce what M2V format is:
M2V file is a video-only MPEG-2 file. It contains MPEG-2 video only, without audio or subtitle. M2V files are used in situations where you need to separate video and audio tracks into separate files. For example, some video editing or DVD authoring software edit video and audio separately, and combine them together in the last step.