There are a number of valuable resources on the web that can help practice the voice parts.
Choraline.com is probably the best known and they have both on-line and CD rehearsal material.
John Fletcher Music is also worth looking at, although you need to register on the site first. 'Out of copyright' pieces are free, others need a 'paid for' membership.
Choralia have a wide catalogue, all freely available, although a donation is requested. The pieces are played on a synthesizer. The parts are given in three levels: the first an individual part with metronome clicks to help learn the tempo, the second with all voices present but the individual part emphasised and the third with all parts at the same level. (Some tried seem incomplete).
Singingpractice.co.uk have a selection of Midi files for 52 different choral works. They also have a selection of carols mainly from 'Carols for Choirs 1' and '100 Carols for Choirs'.
Chord Perfect use real voices (rather than instruments) with each voice part emphasised. They have a limited number of works, mostly free of charge.