When you’re rehearsing a show, it’s important to write down sound levels. That way when you bring a sound or microphone in, the level won’t be way too high or way too low. Don’t get too attached to those numbers, though, as they are just a starting point.
There are numerous things that can affect sound levels during a live show, so those levels will be different for every performance.
Actors are not robots
Maybe a performer has plenty of energy on opening night, but by night two he’s exhausted. That exhaustion might manifest as a quieter performance. Actors also get sick or lose their voices sometimes, and they’re counting on you to boost their mic and provide the extra volume that they can’t. If you mix them at the same volume every performance, then you could be making them sound bad.
An audience absorbs sound
Sometimes you can have everything adjusted perfectly during rehearsals, but when opening night hits, everything is all wrong. Perhaps the audience can’t hear the actors over the music, or the actors are getting off of their music because they can’t hear it.
When you fill a theatre full of people, the acoustics in the room can change dramatically. Generally adding a big group of people to a house will absorb sound, so expect to have to run levels hotter than you did during rehearsals The good news is that you probably have a little more gain before feedback to work with now.
Mixing is a dynamic process
Another problem with the “set it and forget it” theory of mixing is that the levels needed during a show or even a single song are constantly changing. Your mix may sound good during the intro to the song, but as soon as the full orchestra kicks in you might need to boost some singers’ mics. Even prerecorded music can be too loud in some parts and too quiet in others (consider using a compressor to help smooth out the volume).
Unlike lighting levels, sound levels can’t always be programmed or automated. If you have any sound going, you should be constantly listening and making adjustments if necessary.