As Stan correctly points out - you must do this outside of Encore.
As a general guide to setting the correct level for menu audio, try this:
Open the audio file in a media player, and set it to loop.
Play continuously until you have adjusted it to a level that does not irritate the hell out of you, and then turn it down another 3-6dB.
Menu Audio should peak at absolutely no higher than -10dBFS to my mind.