If I write, and publish sheet music for, a hymn under CC BY-SA, and it is sung in a church, and the church records their services, does the recording have to be CC BY-SA? If so, can I avoid this while still keeping modified versions of the sheet music under BY-SA?
Whilst I greatly respect my colleague Philip Kendall, in this case I disagree with him.
There are several sets of rights to be considered, here. There is the copyright in the original composition, there is a performance right, and there are recording rights (and those recording rights, as Philip rightly points out, vest in the producer of the work in the absence of any agreement to the contrary). These rights are not mutually-exclusive. That is to say, when it comes to distributing a recording of a performance of a copyrighted musical work, the interests of all rightsholders must be taken into account.
In the UK, PRS for music comprises the Performing Rights Society, who deal with the original author's interest in performances, and the Mechanical-Copyright Protection Society, who deal with the original author's interest in recordings. Both these schemes are voluntary; that is, they collect payments on behalf of original rightsholders who have joined them, and distribute them thereto. If you haven't joined them, your rights cannot be licensed by a PRS payment.
So it seems clear to me that the rightsholder in an original composition does, in the UK system, have an interest in a recorded performance of a work. I am aware of no analysis of whether those rights can be licensed copyleft-style, but I can also not see any obvious block to them doing so.
However, your case is interesting because you don't want to enforce those rights. In that case, I'd add an exception to my CC BY-SA licensing statement along the lines of the kernel linking exception:
NOTE! This copyright does not cover performances and recordings of the work. I, Pat Someone, disclaim any copyright interest in such performances and recordings.
Note that someone with musical training could transcribe your work from such a recording, and since that would be a derivative of a work in which you had disclaimed your interest, you would have no control over that transcription. Hopefully, this is not a risk you're worried about.
A separate copyright subsists in the recording than the sheet music. The Copyright, Design and Patents Act 1988, the main piece of primary legislation around copyright in the UK, contains specific rules around who is the author of a creative work (Section 9):
(1) In this Part “author”, in relation to a work, means the person who creates it.
(2) That person shall be taken to be—
(aa) in the case of a sound recording, the producer;
"Producer" itself is defined in Section 178:
“producer”, in relation to a sound recording or a film, means the person by whom the arrangements necessary for the making of the sound recording or film are undertaken;
As the author of the work, the producer can license the recording under any terms they wish; they are not bound by your copyright on the sheet music.