I also experienced Siri talking at what seems like 1% volume recently, out of no change that I can recall making. Just learned how to fix that through these comments, thanks.
This has been driving me mad. It seems certain actions have had all audible feedback disabled, but there's no real sense to which ones.
When I say across a room to set a reminder or add something to my shopping list my Homepods will just silently do so, with no indication but a flash of the screen. I have no idea if it's registered what I was saying or not.
When I ask to turn on the lights in a room, it'll do a bing-bong noise at me to indicate that it's registered despite the fact I can see the lights turning on. It's utter nonsense.
My guess is that the combination of a power button long-press continued by pressing the volume down button is something that might happen accidentally while you have the iPhone in your pocket.
Once Siri is active, using the hardware volume buttons control the feedback volume.
