good point about audio clips. they might give a relative comparison, but you can't be sure unless you know how it was recorded. that's why i documented my process and followed the standards established by the Iron Butt Association (which, iirc, were based on state or fed guidelines for measuring exhaust dB). IOW: a sound clip recorded closer might give the appearance of louder, lower (or higher) tone, etc.
But at least they are something to compare.
On another note (heh), looking for a low rumble from an I-4 might be kinda futile. Like the difference between a V8 and I4 car, the lower exhaust notes tend to come from the twins.