There isn't a reset that resets everything back to "factory defaults". You can reset certain parameters but sounds is not one of them. Changing the engine sound is quite easy so I am curious as to what you've tried that you haven’t been successful.
The procedure for changing the engine sound, after you've loaded the new engine sound onto the LM is on page 14 of the HC manual, the part called User Defined Buttons. I am going out on a limb here and presume you know how to get to the User Defined Buttons Setup screen.
- Tim