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If you could post a screenshot or image of your blueprints or C++ code it would make it a lot easier to help. (If on windows 7 or higher, simply search in start for ‘snipping tool’ and selected the desired part of the screen and save image)
However as I understand it, you want to play a sound when the wave ends? In that case, if your settings of the wave spawn have a set ending (e.g. your settings detainment that a wave lasts exactly 45 seconds) simply play the sound once at the end of that code. If not, then you could possibly set a Boolean to see if there is still enemies spawning and on screen. If both are no, then play the sound.
I hope this helps you, if not, please post a screen 
Good luck
no way. SoundCue can’t contain variables besides settings.
if you need it to play another wave after this one is finished - use sequence node to play them one after the another.
otherwise if you need it for external logics, set delay or timer node for the wave duration.