Which of the following is NOT a use of microphones in stage productions?

Providing visual effects is not a function of microphones in stage productions. Microphones are primarily designed for audio purposes; they capture sound waves, convert these sound waves into electronic signals, and amplify performers' voices so that they can be heard by the audience.

Visual effects, on the other hand, are typically created through different means such as lighting, video projection, or visual set design. The role of a microphone is strictly audio-oriented, focusing on enhancing the auditory experience of the performance rather than contributing to any visual aspect.