Yeah, looks like at one point we had some sort of support for sending notifications when a sound loops, but I don’t think it’s currently hooked up to BP and wave instances are only setting loop modes to either LOOP_Forever or LOOP_Never rather than LOOP_WithNotification.
You could try and add a feature to get notifications when a sound loops – there is some branched code in the xaudio2 implementation that looks like its intending to eventually result in a delegate callback but it’s not fully hooked up. To do that, I’d follow the existing OnAudioFinished code and add case for getting something like OnAudioLooped, and then allow a new looping mode on Sound Waves or some other way to indicate to the WaveInstance that you want to broadcast a notification on loop boundaries.
I’m not 100% aware of the intricacies of FMOD’s plugin with UE4, but something like this should be doable in FMOD (as its doable in UE4 with some code spelunking).
If I were you, rather than play longer files, I’d create a BP mechanism that plays smaller music fragments on bar boundaries (using a timer or a delay node or something). With this, your music won’t get too out of sync, you can restart whenever you need, you can switch to other music fragments, layer them, etc.