Solution! (sort of...) Apple TV 4 - Bose Solo Volume Control
I recently set up my Apple TV with a Samsung SmartTV and Bose Solo 5 sound bar. Everything went smoothly except for the ability to control the volume on the Bose.
I'd liked the idea of controlling everything via the Siri Remote (HDMI-CEC takes care of turning the TV on/off and selecting input, and the sound bar is set up to auto-wake and sleep based on S/PDIF activity from the TV - so, all I needed was IR volume control).
As much as I tried, I couldn't get the Apple TV to detect the Bose IR signal - the progress bar would fill up, regress, then stay blank.
Someone on the Bose support site had the same problem, but got no help - Bose support misunderstood the query to mean the user was trying to control the Apple TV with the tiny, 8-button Bose remote. (You should read the thread if you enjoy Kafka.)
That user solved their own problem by using a third-party remote they'd programmed for the Bose to transmit the "Volume +" and "Volume -" IR commands Apple TV needed to learn.
Let me confirm, it works! I programmed a Logitech Harmony remote to control the Bose, Apple TV picked the IR signals up right away and setup was complete.
Checking around, this issue seems to be very specific to certain brands / IR protocols - I hope Apple addresses this in a firmware update.
And, if you're trying to get your Apple TV to control your Bose sound bar - maybe try borrowing a universal remote for a few minutes!
Apple TV 4, tvOS 12