I’m surprised the Sonos  app can control your Sony soundbar. 
Have you tried re-doing the remote control setup? 

Hi, I’m not sure how to do the remote control setup via the app?

I suspect the term ‘Sony’ was supposed to be ‘Sonos’.

Which Sonos soundbar do you have? How is it connected to your TV?

Hi, it just says Sonos soundbar on it and is about 6 years old. I can check serial number or something else to identify it, if necessary?

6 years old, it’s likely a PLAYBAR or PLAYBASE, which is connected via an optical cable, not ARC, via an HDMI cable to the TV’s ARC port. It should say on the label on the bottom of it. At least, as I recall, both my PLAYBARs did, but I never owned a PLAYBASE. 

In which case, if you go in to the ‘room’ that the PLAYBAR is in, you should see the options to set up the remote. 

Note that Apple TV remotes are sometimes in a state where they get ‘confused’, and have to be reset in order for them to work properly. I’m pretty sure there is an Apple FAQ on that, I’m less sure that Sonos has one, but they might. 

Just for the record (as I recall it, here is a list ( in potential order of release, you may need to check my memory versus Wikipedia) of the various Sonos Soundbars, and their connection methods:

• PLAYBAR           Optical
• PLAYBASE         Optical
• Beam (gen 1)      ARC/eARC
• Ray                     Optical
• Arc                      ARC/eARC
• Beam (gen 2)      ARC/eARC
• Arc Ultra             ARC/eARC

The optical devices, which are only able to work with data from the TV to the Sonos, are controlled via IR, which often requires resetting the remote, via some software setting, to emit an IR (Infrared) signal. The ARC controlled soundbars are controlled by the TV itself, via the ARC connection, which has both the bandwidth necessary, and the ability for two way communication, to function. There are several things other than just volume that an ARC connection can carry. Optical is much less.