Thanks. I tried it, and customizesd the main context menu (hiding an item from main context menu) and the "download as mp3" jump to first item on youtube page context-menu. If you provide more details, I can try it: your Menu Editor configuration maybe. I'm not so familiar with it. Seems you can hide unwanted items from context-menu.
EDIT: I asked its developers if there is any workaround for this. since "download as mp3" item, only adds itself to youtube pages, so their extension miss it (can't check all available domains to collect all possible dynamically added items by extensions on certain ones).