Michael Brooke recently attended one of those The Sound of Music singalong things, and reports on the experience. I don't think I'd want to go to one of these, since I actually like the movie (except for Liesl's singing voice, ewwww make it stop). But it actually does sound like it could be fun, if you could get an audience with the right blend of reverence and, well, irreverence.
The one time I saw Rocky Horror Picture Show with a live audience, it was a pretty disappointing experience: not many of the people there knew the whole drill, so it kind of devolved into everyone just trying to do their own MST3K bit. I suspect that's the kind of audience I'd get with The Sound of Music.