I noticed that when adding a drum machine app to a track (in a new or existing project) and then adding an effect (any that I tried) audio output stopped. Exiting the project and reloading would fix it. This happens with Hammerhead, DigiStix 2, SDS-x, Pure Acid, Ruismaker Noir and a couple more. Also, with one of these apps and an fx app loaded and working properly, deleting the effect will kill the sound. Again, restarting the project will fix it.