I agree overall, but I think resonance can occur at a bit above a whisper but pretty decently below normal conversation volume given resonance is more about specific frequencies than actual loudness even if resonance can increase volume. For example, you can whisper in a way you don't feel your vocal cords vibrating and therefore isn't causing resonance in your oral or chest cavities, but you can also whisper to a point where your vocal cords vibrate and resonance can occur though it may not be sufficiently forceful enough to hit the specific pitches necessary for the spell, but I don't think you have to go too far beyond that to have a wide range of pitches and can resonate. Which even humming I can feel resonance in my chest and lowish volumes. 

But I also realize the specific cut off will be widely variable, and I understand why you'd rule it that way and would be fine with it too as a player.