You know, there is a reason that you have preshow announcements telling people not to make any type of visual or audio recordings of shows. Unless the show is an original piece and you have permission of the playwright it is very illegal to record, let alone sell recorded copies of productions. Yes, this means that if you don't buy the rights from the licensing company any recordings and distribution of such recordings is illegal. Not only do you need to buy recording rights, you also need to have all of the visual designers give an OK (for video recordings) and the sound designer give an OK (for audio recordings) otherwise you still run into copyright issues.
It is ok for a
theatre to make an archival video, but that is all it can be. You cannot duplicate, distribute, or sell copies. You can use it as a tool to teach
blocking to understudies or for SMs to check up and make sure that the show is still running the way the director and designers intended.
In fact, the
AEA has sent out memos to members saying that even having a fellow cast member make a sort recording of you singing your song on
stage with your crappy little camera is illegal.
As for school band and choral concerts it is much less of an issue.
EDIT:
Also of note, if you work with union cast, there are even further issues with making recordings. You know those "original cast recordings" of shows you can buy? At a minimum, cast members are making a weeks pay for each day they spend in the studio on account of they don't get royalties from album sales. For in-theatre recordings, even archival, there are rules about how far in
advance you need notify cast members how long a segment you can film, how it can be used,
etc.
In general, it is important that you make sure that you look up all the rules pertaining to your situation before you start making recordings.