It's been out for just over a year now, with the latest version (
VSM Symphony of the Skies v4.0) more recently added. I only just discovered the mod this morning.
This is, by far and away, the most competently assembled sound package I have ever heard for this game. An extremely well-made effort.
I could go on forever about how impressed I've been with it, but it's best if you just heard for yourself.
Don't think. Just download it now.
What are you waiting for?
You'll see lots of options, but all you need to do is scroll down until you see "Symphony of the Skies". Download the 3-part "Samples" packages, the "Presets" package, and finally the "VSM v4.0 Improved Engine Sounds" patch that follows. Make sure your [my-samples] and [my-presets] folders are empty before unzipping and copying.
Should work for either vanilla or any modded version of IL-2, as long as the aforementioned "my-" folders are present in the game's directory.