I've been working on a top down racing game and I have about 4 files of engine sounds to vary different speeds and 4,5 files with gear change simulation. Right now I am only using gear changing at the start and end of races , the rest of the time I am simply looping those 4 different engine sounds with a kinda trancy music on top and I may just leave it like that.
Without the music, though I get exactly what you are saying. Sounds bad and I think the only way to do it is with different files. I don't think Construct's audio manipulation is sophisticated enough to manipulate a single file ( maybe I am wrong, should play around with playback rate to find out )
In older flash games, I had a setup with one file for each gear and switching gears sounds, so just trigger each depending on car's speed. But you have to create those engine sounds that raise in pitch and other details.
I think having a nice soundtrack on top and engine sounds in the background works well and is far simpler, and who doesn't like good driving music?