There are many ways you can handle this. Unreal has a class called FMath which has various interpolation methods like lerp or FInterpInto. Similar methods are also availables for vectors/rotators/matrices/quaternions/…
In your case you can define a float variable called InterpolationDuration (or any other name) and another float called CurrentTimeProgress, then at each tick if some condition is met you add delta time to CurrentTimeProgress or if the condition is not met subtract it and then use FMath::Clamp to limit it between 0 and InterpolationDuration. Also all of these methods are accessible in blueprints with the same name.
After changing the CurrentTimeProgress you Just use lerp between your turbo value and 0 and put CurrentTimeProgress/InterpolationDuration as the alpha parameter.
for the sound there is this whole meta sound system you can use but it is kind of hard to explain here. You can check the official UE documentation on it here: