They're teaching it wrong. I mean, they have to be for every sound engineer I've met to get it wrong the first time. (By every I mean every one but you, of course)
Oh... hi. Let me catch you up on this rant that's been going on in my head for about 4 years now. It seems any time I work with a new sound engineer, they insist on assuming they'll know how to mix the vocals better than the band. Last week's Cirque du Mac party was no exception (yeah, I know, I didn't blog about where or what I was going to be doing at Macworld Expo and, yeah, I've been ignoring my blog in general for a while. I've been busy but I'm doing fine, thanks for asking. I'm here now!).
At Cirque, the Macworld All-Star Band had 5 vocal mics on stage: 3 across the front, one with Chris at the keys, and one at the drumkit for me. With that many live mics on stage potentially allowing other sounds to bleed in when someone isn't singing into each, a sound engineer tends to get antsy that they won't have full control over all the sound. The oft-perceived (and ill-conceived!) solution is to bring vocal mics up and down only when they're needed. On the surface, this sounds very reasonable. The issue is the dangerous assumption by the sound engineer that they will somehow magically know when each mic needs to be on. Of course, it's impossible for them to know this if it's the first time they're doing sound for the band. Hell, it's hard for an engineer to know this even if it's his/her 100th time doing sound for the band. If you have a band that liberally employs harmonies, you simply don't know when one is going to crop up. And by the time you notice someone singing into a microphone, you've missed it. If you're lucky, you'll see it during the first verse/chorus and maybe get it right for the second one. But with 5 mics on stage, you've got to watch each and every one 100% of the time to even get it right the second time around, and I posit those chances to be slim, at best.
The solution is simple: leave the damned vocal levels alone and let the band mix on stage just like they're used to doing in a rehearsal room. Most bands are better at this than most engineers give them credit for (bad English, good statement!). Of all the bands I play in, all the musicians I know are smart enough to back off the mic when blending a harmony and get up close when singing a lead. Mic technique is just one of those things you learn. Trust the musicians. Hell, it's their gig and their reputation on the line if they sing harmonies too loud or out of tune. Let them fail and take the blame, if you wish, but in most cases they'll sound better than you think.
So if you're a soundman reading this, please -- for the love of all that is good and pure, PLEASE -- when mixing a band for the first time, set the vocal levels in the monitors (all the monitors!) the same as the mains and simply leave it alone unless one mic is much louder or softer than the others. Then adjust it in both places and get out of the band's way!