In the last few days, the beep sound is missing from iOS dictation (but not macOS dictation). When I tap the dictation button in the iOS keyboard, I expect a beep sound to indicate it is listening, as it has been for many years. But it no longer happens, I have to watch the screen to see when it begins and ends listening. I have tried rebooting the phone. All software is up to date. System Settings, Sounds & Haptics, Ringtone and Alert Volume Change with Buttons is disabled (I keep it disabled specifically to avoid this problem).
An answer is found! If I tap the dictation button in the iOS keyboard, and then immediately press the phone’s external/physical volume up button, that revealed the volume of the dictation “listening” beep had been decreased to completely silent, but it is now lifted to an audible level.
This was unexpected because the general-purpose volume was already at a mid level, and the System Settings volume for notifications was also at a mid level. Apparently there is a third kind of audio volume in iOS.