Let’s say I’m trying to edit a music clip and make a smoothly repeating ringtone from it. The obvious way to choose the edit endpoints would be to select a portion that seems close, click Shift+Play, listen thru the break, and then adjust the selected region accordingly - except that nothing I try during the playback affects the playback itself. Is there a way to make this sort of dynamic playback adjustment that I’m not aware of, or am I doomed to always have to stop playback and restart it from the beginning of the (newly adjusted) selection?
Adjusting the loop play region “live” is a feature request.
Gale
So you’re saying that I’m not just incredibly dense, and the reason I can’t figure out how to do it is that there isn’t any way to do it yet? Bummer. 
Do I have to take some sort of additional action to turn this conversation into an “official” feature request?
Correct.
No, your vote will be counted.
But if you find another missing feature you can post a request for it here http://forum.audacityteam.org/viewforum.php?f=20 .
That way, it will be certain to have a vote counted - there is a chance your vote may not be counted otherwise .
Gale
A little “trick” that might help:
If you have a piece of music and you want to loop a section of it, then the end of the loop must match up with the start of the loop. Checking that these match up can be time consuming because you have to wait for the loop to play all of the way through before it starts again. However, in most cases, if the end of the loop section matches the start of the loop section, then the end of the first bit before the loop will match up with the start of the bit that is after the loop, and this is easy to check.
Make the selection as close as you can by eye, then press “C” on the keyboard. This will play the last few seconds before the selection and the first few seconds after the selection. You can set how many seconds play in “Edit > Preferences > Playback > Cut Preview”. I usually set this to “3 seconds before” and “2 seconds after”.