In case someone is not aware of, I would recall that there are a large number of MIDI versions of Bach works in the net, especially in www.prs.net , including instrumental and choral music. The Passions, Magnificat, Christmas Oratorium, and literally hundreds of pieces are available for students.

We at our choir have had excellent experiences in learning complicated choral works by the method of recording a CD for each voice (one for Soprano 1, one for Soprano 2 ... one for Bass) including all voices but enhancing one in particular, so you can study your part reading the music sheet and listening to your voice well differentiated among the others and the instruments. Many of those versions include all tempo and volume variations (crescendos, diminuendos, stringendos, ritenutos,etc.) and this way you can obtain a fast learning of the notes and rythm, and then dedicate most of the time to polish expression.

Un abrazo - Andrés