I don't want to do anything invasive like actually track visitors, I just want to be able to offer helpful "You're near X exhibit" type notifications. Obviously if there's a group close together than it may just become annoying/unnecessary - I'll have to figure that out as I talk to the curators.
QR codes are a good backup and would mean that they wouldn't need an app at all (although I would love to build them one!)
Thanks so much for sharing your experience. I'm thinking less about mapping (at least, not 'live' mapping) and more about proximity to key exhibits (ie the app says 'hey, you're near X painting' and jumps to the info on that piece. Some of the technical solutions you outline are really interesting (background music!). It may all come to nothing but I'm enjoying investigating and talking to other museum people :)