Choral Public Domain Library

https://www.youtube.com/watch?v=1umvRRm5Eg4

The viewing option shown at around 0:30 is very useful for seeing one or multiple voices simultaneously to check on and identify the source of tuning issues.

(I have no affiliation with whomever makes this app; it's just fascinating).

https://play.google.com/store/apps/deta ... c.pitchlab

This app has come to semi-affectionately be referred to as the "bitch slap" application by choristers who don't love being told that they're the voice part or singer that's going flat.