After replacing, you need to add audio track info and enable it. Go to dummy DVD, Menus domain, Audio Tracks pane, highlight the audio track and select "Copy tracks info". Then go over to the project where you just replace with the motion menu, Menus domain, Audio Tracks pane, and "Paste tracks info". Now go to the menu PGC level, Audio Tracks pane, double click on Track 0 and enable it. You should be able to hear sound when playing in the Preview window.

Don't worry about the error, it's a warning that different streams have been detected when you're replacing the block due to added audio stream.