That's a damn good question... I scanned the cache directory and didn't see anything for jeep. Or for M16, for that matter...
I notice that TwinBoom's soundpack simply uses "jeep" as the directory name. Are you sure it's not just your sound file, rather than the directory? Copy a jet engine sound or something really noticable that you know works, and test the jeep out. If it sounds like it's got a jet engine, you know the directoy works, and the file just is the wrong format.