Fellow Kerbonaut KingPengumon had an interesting idea on Forum:

hi does this work with TweakScale? Like scaling the pitch and maybe sound if you are tweakscaling the engine size.

The code should not be that hard, essentially an audio filter to be added to (or replace?) the original sound. The hard part will be doing it in a sensible way but, hey, it may be fun!

I don't have the slighest idea when I will work on this one, but at least I will not forget about it neither!

The "problem" with sounds is that they don't "scale" exactly (no one wants a 186 dB sound effect coming from their speakers!) - I would need to change the pitch, but also how far the engine can be heard.