I like to be able to text silently (that's the whole point) and I often find myself hitting the mic. button accidentally, since it is RIGHT NEXT TO THE SPACEBAR! (Why?!)
Is there a way to turn it off so that I won't get the disturbing beep every time I hit it by mistake?
Only by turning off Siri (and this will be universal, not just for the Messages app): Settings App > General > Siri).
Yes - turn Siri volume down.
Hold home button down until Siri pops up, then immediately hit the Volume Down button. The lower you go, the more quiet that beep prompt is - both with Siri, and the Mic button on the keyboard.
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