First, create a sub-folder in HTC->Aces High->Sounds. For example, c://HTC/AcesHigh/Sounds/P51D. Then, copy the sound into that folder and name it using the names listed in the AH "Help" pages. For engines running, its "eng.wav". All "engine running" sounds have the same name, but are differentiated by having them in different folders. When the sim starts, it looks for the "eng" name in the folder for the plane you are flying.
So, you would have an eng.wav for the P-51D in the P-51D folder, and then you could use a different engine sound called eng.wav in the FW190 folder.
Make sense?
What I usually do is copy the sound directly into my C:/ drive, then open up "My Documents" from the desktop and look for the file on the C:/drive, right-mouse-button click and select "Copy", then open up the C://HTC/AcesHigh/Sounds directory and find the folder for the plane I want to modify, open the folder and paste the new sound in. Then, I delete or rename the old "eng.wav" file, then rename the new sound as "eng.wav".
The sound volume can be adjusted using Aces High's Clipboard in the game itself.