I have a solution. There are two settings that need to be tweaked. The first is found here... Settings->Sounds and Vibration->Separate app sound Originally, this was set 'On' and as soon as I turned it 'Off' I was able to hear music on my headphones. The second setting is here... Open Samsung SmartThings app On the top right of the screen, there are 3 dots, tap them, and choose 'Settings' At the top, there is a setting called 'SmartThings Panel' if this is off, turn it on Once it's on, a little panel is displayed, if you have 'Separate App Sounds' switched on, there will be a little speaker icon on the SmartThings panel, which lets you choose which output device to use. You should be able to turn the 'SmartThings panel' off once you've got it working, but I needed to turn it on to select my headphones.
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