It's important to understand that the sound control isn't "reversed". It's technically doing everything correctly, even if that "correct" is wrong.
Engine volume is based off of how high your current speed is compared to the current max speed, including ETS adjustment.
Basically, imagine your ETS is normal, and you're at full speed. 50/50 speed, engine sound at 100%. All is good in the world.
Then you decide to crank up Engine power. The top speed is now 62, but you're still at 50m/s because obviously you need to accelerate first. The engine thinks "Oh, the pilot isn't at full speed yet, we can't play the engine sound at full volume until he reaches 62!"
The top speed has changed, therefore the range of volume has changed.
So, you're now at 58m/s, still accelerating. The engine volume has started increasing as it should. But now you divert energy away from the engines. Your top speed drops down from 62m/s to 35m/s. Which means you're way over top speed now, because you were flying at 58m/s. The sound control thinks "Okay, the pilot is flying at or above max speed now, let's crank up the engine volume to max." Despite lowering power, the volume increases, because the game thinks you're at top speed.
This is the whole issue here. You think the solution is simple, but it really really isn't. It'd probably require a full rework of how the engine sound volume is handled, and there's no single operation that can be flipped around, because the math is correct, it just doesn't handle sudden changes of top speed.