Well, true sound is available for some models and it isn't done with speakers. My T-33 sounds almost exactly like the real thing. There are true turboprop engines available for models as well. Unfortunately, this is probably about the only way you will ever be successfull creating a truly authentic sound for a model airplane.

I have often had thoughts of building a 1/72 scale aircraft carrier where the propellers would spin on the planes and play back the sound of many engines starting up. Actually did build a carrier but gave up the idea of the engine sounds out of pure frustration at even finding anything at all to accomplish it.