This subject has been up before, but the short version of it is that the planes we fly have “constant speed props”. You set a control for the rpm you want, basically hi rpm for hi performance and low rpm to save fuel. This is achieved by a governor that tries to keep the rpm constant by changing the pitch of the prop blades. (I think the only exception in the current plane set is the 109, I think the pitch was controlled directly by the pilot on them.)
From when the engine is idling on the ground increasing the throttle directly effect rpm and the prop is as ‘flat’ as possible until the rpm reach the level set by the pilot. At that point the prop governor starts to increase prop pitch, increasing the load on the engine to hold the rpm at that set level. The prop governor can not keep the rpm perfectly though and would take a little time to settle after a quick change in manifold pressure or rpm.
The point is that the engine sound should not change pitch under normal flying conditions with constant rpm, but the volume should increase with manifold pressure and the characteristics of the sound should change with engine load, like in a steep climb the sound should get ‘rougher’.
-Jinx