OK. A quick How-to on changing how the sounds "appear" in the cockpit, vice the outside view.
I apologize if anything is over-explained, or stuff you already know, but better too much info than not enough.
First, always, ALWAYS make a back-up of ANY file you're going to edit BEFORE you start. This is vital, in case things go totally south, so you can return things to a starting point. Make another back-up for every time you change something, so you don't have to start over from a point long behind you.
Second, ONLY open .cfg files with NotePad, included with Windows. Other word processors add symbols to the files which make them useless to the sim.
Go to the \sound folder of the plane you want to change. Like D:\steam\steamapps\common\FSX\SimObjects\Airplanes\FA-18C_FSXBA_BA_17.1.31\sound , just for example. Open the sound.cfg file. Here's where it get's complicated, although not too much.
Look down through the file for the kind of sound you want to change. You specified the jet whine, so in this case, look down through the various sections for things like this: [whine_12]
Under that heading, look for a line that reads: viewpoint=1 Note the number 1. This indicates "inside the cockpit" type of view point.
Once you've found a section with those two conditions met, look for the line in that section:
vparams=0.00,0.00,0.21,0.00,0.27,17.00,0.35,18.00,0.40,7.00,0.50,0.00,1.00,0.00,1.00,0.00
This line is made up of 5 pairs of values. The first in each pair is the point in the variable's range and the second is the relative sound volume.
The second value is a percentage. Anything over 1.0, or 100% is read as a 1, again, 100%. Can't go higher than that. Change the second value of each pair up by a certain amount. Say .05 or there-abouts. Only change ONE vparams line at a time, so you can keep track of the changes you've made.
NOTE: you can play the sound the sound.cfg calls out to see if is what you might want. In this case, the file called for in the specified section is filename=x_whine_12 . Just look in the \Sound folder for that file, and double click on it. Windows will play the sound file with that name for you, so you can hear it. Handy to preview a sound file to see if it's the one you want.
Save the file off, reload the plane, and test to see how it sounds to you. You can close the file, or leave it open for editing, as long as you make certain it gets saved off.
A good way to reload the plane is to assign a key to "Reload User Aircraft".
NOTE: If you are going to use this key assignment, you need to make sure ALL AI traffic sliders are full left, or zero percent. If there are AI craft present in the world, whether aircraft, boats, cars, whatever, it can have haphazard, unpredictable effects on the reloading of your aircraft.
Another way is to select another aircraft from the normal pull-down menu. Not just another paint of the same plane, but a totally different aircraft. Like go from the F/A-18 to a Cessna 175, and then back again.
Finally, you can also select End Flight, let it exit out to the Free Flight screen, and then back in again.
This takes a little time to accomplish, but it will achieve the desired results.
Sorry I took so long to get it posted. Real life interferes...
Good luck, and enjoy!
Pat☺