Taken from
http://www.waf80.de/virus/faq.html -
>OK, this has gone on long enough. To do vocal choir type sounds well, you need at least three tunable highly resonant filters. You tune them to form whatever vowel sound you want. The Virus only has two, so it will not be so great at doing this. If you have additional hardware like MAM or Doepfer triple bandpass filters, they will help a lot. A parametric EQ will help a little bit, but they don't allow enough of a Q setting. A vocoder that has a freeze function can also be used, and then tailored with additional filtering. Robert Moog wrote some articles on vocal simulation 10 to 20 years ago in Keyboard magazine, if you have access to those. Also the German Doepfer website has the online manual for the A-127 triple bandpass filter showing frequency settings for different vowel sounds.<
First prize goes to the most convincing choir sample!