You can make your own. Just find a .wav file you like and save it, then rename the old file (in case you want to swap it back later), then rename the file you just downloaded as the original file's name (such eng.wav for the engine).
For example, let's say you want to change the N1K2 engine sound to be a TIE fighter from Star Wars. Just d/l the .wav from the Internet (let's call it TIE.wav) to your computer. Then, since you have Mitsu's soundpack, each airplane will have its own sound file. Just open C:/HTC/AcesHigh/Sounds directory and look for the N1k2 folder. Open the folder and copy TIE.wav to the directory. Click on the eng.wav and rename it eng-original.wave. Then, change the TIE.wav file to eng.wav. When you start Aces High and select the N1k2, you will hear the TIE engine (after the engine start sequence is completed).
