I imagine passengers will be exposed to very high noise levels during flight.

I don't have a good intuitive feel for that.

At 25km altitude, with 1% of normal atmosphere, maybe you're close enough to vacuum that it can get really quiet?

	▲	pdonis 11 hours ago \| parent [-]
		The engine noise can still be conducted through the body of the plane. With the kind of ramjet being talked about, I think that's still likely to be significant even at very high altitude.