After carefully listening for a couple of days to MP3's made from a WAV file recording of a brass, choir and organ rehearsal, I developed some rules of thumb about bit rates. I concluded the following minimums are indistinguishable from WAV files in capturing the music of their various types. They are expressed in the usual thousands of bits per second, Kbps:
Speech (without too many resulting artifacts) needs 56.
(Hymns with) descant need 64.
Brass instruments reduce the needed rate for choirs to 64.
Choir a capella (they are rather pure tones, or maybe it is the harmony) needs 80.
A single cantor (alto) needs 128.
Propers (increased because the choir response pitches are slightly clustered) need 160.
Hymns (depending on the organ stops) need 64 to 192.
Organ (because of reed and string stops) needs 192.
(The tests were done using Audacity v. 1.2.6 and MP3 LAME v. 3.98.)
BTW, Apple's iTunes Store uses 128-Kbps AAC (Advanced Audio Coding) format, claiming equivalence to 160-Kbps MP3.
Edited April, 2012 to include AAC.
Copyright (c) 2009 Mark D. Blackwell.