I agree with Mark on this one. Sound cards in computers are not the most resolute in terms of D/A conversion. They are not made for critical listening, unless they are the highest end soundcards(and even those cannot keep up with most mid to high end DVD or CD players)

Out of all the music I have recorded in my life, choral music(especially massed choirs)is probably the most difficult to record with a high degree of intelligibility. Choral music contains tons of harmonics and overtones when all of the voices are combined with music. It takes a dang good D/A converter to sort all of this out, and dang good speakers to resolve the complex timbres. If you want to get to the bottom of the problem, be open to all aspects of your system. Especially your sound card.