In AE CC 2014, the audio previewed while using Motion Sketch. This was super useful for timing, but now it seems to no longer be an option. Is there something hidden that switches that feature back on, or has it been completely discontinued?
I haven't used Motion Sketch in forever. I don't have CC 2015 in front of me to test right now, but it's possible that got lost in the transition to the new architecture. If so, file a feature request for them to bring it back: Feature Request/Bug Report Form
In the meantime, do your Motion Sketch work in CC 2014. Project files are compatible between AE CC 2014 and AE CC 2015 with no issues. If you don't have CC 2014 anymore, install it! How to Find and Install Previous Version of Adobe Apps in CC 2015