You bring up an excellent question. I too wish there was training resource out there for mixing musicals. I don't know your experience or what type of environment / setup you are working in, so feel free to ignore anything I put down here if it doesn't apply. I am also not a full time professional in the theater field unfortunately. So - feel free to disagree with anything in my post here if it's not correct.
I find myself often riding faders when mixing musicals. Not necessarily faders on individual channels (though sometimes those need adjustment) but moreso DCA and group faders. For example, at the end of some songs the music mix comes up for the finish (maybe that's a pre-recorded
track or maybe it's a pit band mix). Or maybe on some number everyone onstage is singing at the top of their volume and the SPL goes way up, so I bring all mics down just to keep the whole mix from getting way too loud.
Some professionals mix musicals using VCAs / DCAs and
dynamic DCA assignments. One theater I work at just finished up with "Young Frankenstein, the Musical" and I mixed the show this way on a digital
console.
Since I have been mixing with DCAs to mix musicals, some things made a big difference for me over mixing musicals without them. 1). I could more easily manage the chorus levels 2). I could more easily pop people's mic out if they had a feature
line in a song (or
duck the other mics under - and 3). I wasn't as worried about turning on and off mutes for 20+ board channels anymore (because of the DCA assignments) and this made some things much easier and less stressful for me. It was also easier to mix the show overall.
I'm not sure if there is a dedicated training resource for how to mix with DCAs, but there are videos showing people mixing with DCAs in different ways and also I was able to glean a lot of info by watching videos of some software products being used. For example, James Holt's X32
Theatre Control. Eventually I was able to create my own software to automate DCA assignments on a
mixer I use fairly often.
One thing that had frustrated me in the past when I was trying to mix a complicated musical was that there just wasn't enough time to learn the mix during tech week. So I started multitrack recording the rehearsals to computer via my digital
console and then would try learning the mix offline by playing the recording back. It's something that has worked for me in the past, and maybe it would work for you too.
Another reason I started multitrack recording rehearsals is that the Director,
Producer and Music Director could never give me a complete or honest answer as to who was singing in what chorus and they would make certain tweaks to the show, like some chorus members would have to leave the
stage early for a costume change,
etc. The recording gives me some way to confirm these situations so I don't have someone's mic on when they are in a costume change.
Also - a while back I picked up the
book "Mixing a Musical: Broadway Theatrical Sound Techniques". I had hoped to learn more about the actual mix performance and maneuvers, sound levels & metering, and strategies to assign channels to groups, and possibly even little tricks like the so-called "broadway bump" and what not. But, this particular
book left some to be desired on those topics unfortunately as there was one chapter, chapter 15 I believe, that had some of this content. However, I still recommend the
book, as there is a lot of other valuable information in it.
Here are some videos you might find interesting. Again, they may not give you the complete picture of a musical mixing process but it's a start.
https://www.youtube.com/watch?v=b9tuiNVi9AY