QUOTE(Nooch @ Nov 12 2007, 12:29 AM)

Does the kulah of the Seridei Aish include a situation where you know the women in question and can see them while singing? Or is this specifically when you can not discern who is singing what? Does his kulah even include women singing on a different key (harmonizing for example) where the woman's voice now becomes more distinct and recognizable? Because typically a man and woman singing while singing the same notes are on different scales.
I will let you make your own judgment call about the limud zechut of the Seridei Eish. The famous teshuvah is 1:8 [although there's also relevant comments in 2:9]. If you need it scanned, let me know.
Here is a selective snippet:
"...As far as the custom of men and women singing religious songs together, when I came to Berlin and saw men and women in devout homes singing sabbath melodies together, I was shocked, for it goes against an explicit law. Since unmarried women are assumed to be ritually impure, it is forbidden to hear them sing. On these grounds, I pretested against this custom of religious german jewry. After careful investigation, however, I became aware that both R. Azariel Hildesheimer and RSRH of Frankfurt had allowed men and women to sing such songs together. Someone told me that the reason for this is that "two voices cannot be distinguished." Since they sing together, ther is no sin. Even so, my mind was not at rest. I then searched and found sdei chemed who quotes a sephardic rabbi who allowed mixed singing of reelgious songs. I also found explicit scriptural support for this view in the verse, devorah and bark ben avinoam sang". Rambam wrote "...even to listen to a woman sing ...is forbideen". This implies that only if one listens for pleasure is it forbidden, except when reciting the shema. ...we must therefore conclude that with religious songs, men will not have the intention of deriving sexual pleasure from the women's voices. Such songs arouse a holy feeling, not erotic thought...Here, since the men singing religious songs with the women have the lofty intentions of arousing the women's religious feelings, and planting in their hears a love for our holy faith, we can rely upon the lenient view...."
As for your question regarding different keys, I have no idea, but it seems to me to be a red herring since one of the primary sources for trei kalei la mishtamei is the gemara in Rosh Hashanah regarding the shofar that was blown at the same time as the chatzotzrot, and I seriously doubt that the sound from a short, bent, organic, shofar would be the same as that as from long, straight, metallic chatzotzrot, yet nobody argued that because the sounds are different that trei kalei wouldn't apply. Rather, on Rosh Hashanah one had to stop the chatzotzrot short and continue only with the shofar.
By the way, Nooch, in your home, what are the expectations of female guests during shir hamaalot and bentching? Do they need to remain silent then as well?