David Hopkins has pointed out another option: there are several recordings of the work in MP3 format that can be purchased (the typical price is US$9.99) and downloaded from Classical Online. Or you can buy a CD from Marbecks.
For note-bashing, I find MIDI files the most helpful. As usual there are a few different ones around, and some will suit your preferences better than others. The most popular amongst Capital Choir members seems to be Cyberbass, where the MIDI files are embedded in a web page and can be played via a plug-in. I’ve been unenthusiastic about Cyberbass in the past, but I’ve tried again for the Beethoven mass and have to admit it’s pretty good. For each movement there’s a MIDI file in which all the voices are given equal weight, and then a separate file for each part, in which that part has been emphasised. If you want, you can purchase a CD of the whole work for your specific part for US$25, but playing it through your browser works pretty well, provided you have Internet access, of course. Alternatively there are several sites from which you can download MIDI files. Quality varies widely and many sound pretty awful. I’ve listened to a few and the sets I prefer are on the Learning Choral Music and Royal Free Music Society sites. However Cyberbass is what I use these days.