I don’t often use Edge, but found the same result on an older version. I’ve had mixed results with FF: no issues with audio, but it doesn’t seem to respect the stop time on my desktop or mobile. I have a web standards friend at Mozilla, so I’ll see what I can do to have that tweaked for everyone.
While it’s very nice to have both stop and start, I more often find that start is typically enough in most of my personal use cases. I suspect that Hypothesis would prefer the stop too though!
P.S. I’d manually sent my reply to your site, but it may have been caught in your spam filter (for all the links, WordPress doesn’t like more than 2 as a default) http://boffosocko.com/2018/01/07/reply-to-annotating-web-audio-by-jon-udell/#Syndicated+copies+to