At every concert or show, we want the audience to perceive the music in as natural a state possible. There should be no distortion or transformation of the sound, and all the instruments and vocals need to be properly balanced. Not everyone hears music the same way, and there can be big differences in how the music sounds in different parts of the hall, so the job of the live sound engineer is to create a mix that will have all the instruments at the proper level and the overall volume of the music to be comfortable to the listeners.
At the same time, they must create a separate mix for the performers on stage. Sometimes, there’s a separate mix engineer on stage dedicated to only the monitor mix for the musicians. With in-ear monitors, musicians may also be able to tailor their own monitor mix in real-time during the performance. Having a proper Sound Check is often the key to having an enjoyable performance for all.