I'm not particularly interested in digging through console logs to see what was happening but suffice to say it was related to the renamed file extension. One possible clue: When I had the renamed (m2v to mpg) file saved in the session, there was a looooong hang launching Reaper. But I'm interested if anyone knows the nuts and bolts of what's going on here.

I'm certainly not asking for special video features out of a DAW. How to Convert MPEG/MPG to MP4 Launch the Video Converter Assist program. OK, so I have a workaround (convert to mpg). Just saying this suggested workaround didn't work. I'm not disagreeing with '.m2v is an MPEG2 format videostream'. Select MP4 as the format you would like to use. 'Convert' it to mpg and it's perfect again. Your MPG file should be selected as one you would like to convert into a format known as MP4.

As you see, other popular video formats and video presets for portable media players are also provided for your choice. To change WTV to MPEG 1, MPEG 2 or MPEG 4, simply find the format icon you want and click it to set it as the output format. Convert any MKV to MP4, MOV, AVI, FLV, WMV, AVCHD, etc for playing MKV on iPhone, iPad, iPod, Apple TV, QuickTime, Android, Samsung TV, Windows Media Player, etc or editing MKV on FCP, iMovie, Windows Photos, etc. Now choose a suitable output video format from the Output drop-down list.

The video file will still play but at the wrong speed (ever so slightly slow compared to the audio). Convert MPG (MPEG-1 or MPEG-2) to MKV for any MKV compatible device, media player or video editing program. Simply change the extender from '.m2v' to either '.MPG' or '.MPEG' and any software DVD player should play the file - as will Windows Media Player, if you've installed (and registered to Windows) the required MPEG2 filters.'

How to convert mpg to mpeg 2