I know this is a very late answer, but just leaving this here for anybody else who comes across this question.
So because the project is a C++ demo project developed by Epic Games (to show off UE4's C++ functionality), it's not as simple as changing the BP reference to an UAkAudioEvent. What you'll have to do is change the functionality of the C++ files from XCode to directly 'Post Events', or make calls from UE, from within the .cpp files. So set up the C++ project to follow Wwise by doing the standard integration from the Wwise launcher for the plugin, follow this procedure:
After that, go to your BP, which is a data-only BP I believe, then access the 'parent class' from it, if those references aren't in the .h file, it might be in a different .h file, which might be one of the #includes at the top of the document. Have a look around, until you find those UPROPERTY(EditDefaultsOnly, Category=Sound), USoundCue* SomeSound, then find where SomeSound is referenced in the .cpp file of the .h, and then call an appropriate PostEvent() function from the Wwise .h files within the Plugin>Wwise>Source>AkAudio directory. Make sure you #include (at the top of the .h, or .cpp file) which AkAudio .h file you're wanting to use the functionality of, then from there you should be good to go, as long as you use correct syntax, etc.
Not a lot of documentation I could find on using the C++ functionality of Wwise with UE4, so the best bet is use the helpful comments and to look at the code in the AkAudio Plugin C++ files.
Hope this helps somebody, sorry it was typed quickly :)