I have been told by others that someone blowing into a mike to test volume, etc. will damage it. Is this true?
In most cases, blowing a stream of air into a microphone will not damage the microphone, but in the rare case, might damage the loudspeaker. We suggest saying "Test 1,2,3" as a safer alternative.
However, never test a ribbon microphone by blowing into it as the stream of air might stretch or rip the fragile ribbon. Ribbon microphones are now quite rare and mostly found in recording studios.