Power up in safe mode:
With the device powered off, press and hold the Power button (located on the right edge) until the Samsung Galaxy Note 4 logo appears then release.
With the Samsung Galaxy Note 4 logo on screen, press and hold the Volume Down button (located on the left edge).
Continue to hold the Volume Down button until Safe mode appears in the lower left of the screen then release. (Note This may take up to 45 seconds. )
If that fixes the problem, it's an app locking up. (Since it seems to happen when you're listening to audio online, try that. If it starts again, it's that app.)
If it still does it, it's the phone, and it has to go in for repair.