Chord Perfect is FREE to listen online and recommended by Nick.
https://www.chordperfect.com/
Other sites with practice aids and resources that have been recommended by members are:
johnfletchermusic.org
www.learnchoralmusic.co.uk
http://www.choralia.net/mp3catalogue.htm
www.cyberbass.com
www.impresario.ch/choral/