In certain cases this is ok and certain cases it is a big no no. Any light that is functioning with a dimmer will add resistance to the electrical circuit which can cause a loud buzzing in your sound system. This is a common occurance when working in an old building or hotel where you are trying to use their “house” lights that are on dimmers. Whenever possible it is recommended that the sound and lights be run from separate power sources. This can be achieved by using separate electrical circuits when working indoors.