I think the entire muffler fell off, not just the back half (which is how I read this the first time). SharpProp, do a search here and you'll see a post by RCKen showing how to secure the back half of a 2-Stroke muffler. Sometimes the engine's vibration will work the nut securing the back half of the muffer free in flight. You then hear that same noise you described in your first post - yeah, been there, done that.

I use Ken's method of securing it with wire. A jam nut also helps if the bolt is long enough. Of course, none of this helps if the whole muffler departs from the engine

Food for thought ...

-MA