Q: How do I convert an m2v file?

Hey David,

I understand that you are wanting to merge the video and audio from older movies you have made in iMovie. Let's go over some general information to get that worked out. 

You should be able to import those files back into iMovie by going through the import process or just by simply dragging the videos on top of iMovie to add them to the library. Take a look at the articles below for more details. 

Import media from your Mac
http://help.apple.com/imovie/mac/10.1/#/mov6fa10197d

Because the audio is separated, you can add the audio from one of these sources; iTunes, Sound Effects and GarageBand. You might want to add the audio to iTunes and keep track of the names so that way you can easily just drag and drop and the align the audio with the video. Take a look at the next article for more details. 

Add background music and sound clips
http://help.apple.com/imovie/mac/10.1/#/mov91a895a64

The final thing I want to say is that if these are the library files, they can be upgrade to be used with the newest version of iMovie. Then it would be a matter of recreating the video and sharing it. The last articles will help out with that. 

Update projects and events from previous versions of iMovie in iMovie (2014)
https://support.apple.com/en-us/HT202853

Take it easy