When musical bands prepare nowadays they at length state meaningless sounds for the microphone, but in times past they stated something like "one, two, three". Are there good reasons for this change in microphone testing, and when did it start?
The "microphone test" is actually a complete system test designed to find problems in the microphone, cables, amplification and speakers system. Testing one, two, accomplishes the task and requires little thought, but there's no restriction here and any kind of vocalization can accomplish the task. Performers just need to know the system is functional and reliable before they begin their show.
'Testing, one two...' is traditional and still very much in use. Another favourite, particularly in theatre and TV is 'tell us what you had for breakfast?' Actors find it easy and natural to respond to that.
Prolonged sound checks, and that horrible practice of 'ringing out' normally indicate bad equipment, a bad setup, or an attempt to run the show far too loud.
Experience and skill are required. Performers rarely manage to give an idea of performance level in a sound check. Actors are sometimes reluctant to wear a body mic in the position it WILL be worn in. Anticipate the ways in which they are misleading you :-)