dont use flip flop.
instead, set a boolean for when music is active. The toggle can then just be NOT bMusicPlaying = bMusicPlaying
(this just makes the boolean opposite of whatever it is, and then stores that)
the problem with flip flop, especially on a widget, is that it does not remember its own state. so if widget is removed or constructed a new one or whatever, the flip-flop may not actually be toggling. Whereas the boolean on the player controller is cached, so it holds its state in memory.
to make debug easy you can right click on the boolean and set Watch This value. Then slide the blueprint onto second monitor or whereever you can see it while in play mode and then press the widget buttons. You should see the value changing in real time. You can do the same thing with the flip flop node. The execution wires will highlight when they fire. This will help you see what is actually happening.