Well actually forget about the game blueprint. You are already casting to your character i see.
On your character add a Audio component.
Create a custom event in your character blueprint.
Call it Start Music or something like that. Create another one called Stop Music.
Then In your UMG after you cast to you character run the custom event.
And at your custom events you add your logic and then play sound and stop sound.
I can give screenshots but it will have to wait till later today unfortunately. Basically you need to control your music on your character… drag n drop the audio component into the event graph on your character and when you drag from it you will the option to stop.
You just need to set the audio to play by doing that stuff that was in the answerhub post.