For some songs where there are online resources, I have provided links to the appropriate resource rather than a practice track I have produced
The Chord Perfect website provides sung practice tracks with a comprehensive online player (mono). Some pieces require a donation, others are free.
Cyberbass have a comprehensive free online player (stereo) on their site (much like Chord Perfect) or you can purchase tracks for download. All tracks are synthesised
John Fletcher music require you to create a login before being able to access their files, which are synthesised and stereo. Some pieces are free, others require a signup to premium membership. Premier membership is available for 4, 6 or 12 months. If more than one person was interested in this it would be cheaper to purchase a group membership. I suspect this will only give you access to their simple online player. For free pieces, you can access their online player or download the parts. Some free pieces also have midi files available.
Youtube sites vary in quality - some are better than others. Best way to evaluate is to try it
ChoraLine uses synthesised tracks (quite good in my estimation) and vary in price. You can get the 20% Overton discount by entering OVERTON in the voucher box. You can sample on the website before purchase.
I have found 5 sources of practice tracks - 2 are paid, the rest are free.
The Chord Perfect tracks have 3 sections avalable free as samples - if you want all of them, they have asked for at least $10. ChoraLine uses synthesised tracks (quite good in my estimation) and costs £7.99 with the Overton discount (enter OVERTON in the voucher box) - you can sample on the website before purchase.
All the free sources have their parts synthesised rather than sung. The Musipedia tracks are available on YouTube. I don't rate them highly, but you can see the music as the track plays. Cyberbass have a handy player on their site (much like Chord Perfect). John Fletcher music require you to create a login but once you've done that, you can download all the parts from their dropbox account. Your part is differentiated in the same way as ChoraLine but is not stereo.
My recommendation is to try Cyberbass first (unless you're a fan of the Chord Perfect sung tracks). Personnally I'm going to use John Fletcher as the tracks are slightly better and I can put up with downloading Dropbox files.
There are also playlists available on YouTube. Search for "mozart requiem bass" or your part instead of bass. Choirparts (link) have comprehensive playlists for each part
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. this is the bottom of the page