I have tested on my two 1520, same problem. It shows on action center but no sound. I ticked "play a sound" in outlook app, also it is enabled in setting-notification and action.
I have the exact same problem. I've done hard reboots, and even did a fresh install, and didn't restore from my backup and I still have the issue. I'm using a 928. ftr
I fixed the problem by going to mail setting--Notifications and check "Show a notification banner" and "Play a sound". I do not have to do that in the earlier build. Now I have to select both of them to get sound.
For some reason after I first loaded Win 10 and after I did a hard reset a couple of weeks later, for a day or so Outlook would not play a notification sound. after that it magically started to play them. I had not made any changes that I am aware of. I did verify all the settings were correct for sounds and notifications though.