How can I change the volume level of the dictation sound in iOS?
Get the keyboard active in any app or swipe down on home screen.
Press the Mic icon.
You don't need to say something, just hold down volume down button. Press the Keyboard icon to get out of dictation.
It will be muted afterwards. Your music and ringer volume will remain unchanged. I discourage doing this as the phone is then recording your sound which is something the company wants to warn you about.