If you're using Windows 7 N, you need to install a media feature pack to preview text-to-speech voices in Storyline 360.
If the media feature pack isn't installed, you'll see this error message when previewing text-to-speech voices:
The program can't start because MFPlat.DLL is missing from your computer. Try reinstalling the program to fix this problem.
To fix the problem, close Storyline, install the Media Feature Pack for Windows 7 N, then try to preview voices in Storyline again.