I have always wanted to ask that same question because it's obvious that, for the most part, you can't hear Michelle at all and yet, from watching the DVD it's obvious that she's singing along. I heard that the reason you can't hear Michelle singing at Monterey was because the previous 2 performances of the Who and Jimi Hendrix had caused some damage to the microphones and sound system (monitors) since both groups were destroying their instruments which knocked over microphones etc. If you watch the Mamas and Papas come on stage for their set to "Straight Shooter" you can see Lou Adler on stage trying to adjust the monitors and microphones because even Denny's microphone wasn't working at first - and you can actually see him trying to sing into the microphone when he first picked it up and it wasn't working. Obviously, they didn't have enough time to adjust the volumes of the microphones and make sure everything was in working order between Hendrix's set and the Mamas and Papas set. You can here faint snippets of Michelle singing when she's in close proximity of either Cass or Denny because their microphones are obviously working but her's obviously was not working which is too bad. It's a shame that Monterey is the only real live concert recording available because it was - by their own admission - their worst live performance. I can't believe they never recorded other live appearances like Carnegie Hall or the Hollywood Bowl?!
Rob