You could just check frame.hands.length, which will tell you how many hands the leap motion detects, and put a simple if statement saying if frame.hands.length > 0 play audio, else stop audio.
If you're wanting to specifically use the interaction box you can use frame.interactionBox and then the interaction box has a function called normalizePoint which takes in an array position (like palmPosition), and returns a position array with values from 0-1, and if you don't clamp the values then if any of values are less than 0 or greater than 1 then they're outside the interactionBox.
This might explain the interactionBox a little better: https://developer.leapmotion.com/documentation/javascript/api/Leap.InteractionBox.html
Personally I think the first option is quickest and easiest.