Since the TV-to-Playbar connection is via Optical rather than HDMI ARC then the only way you'd achieve an onscreen display is if you were altering the Optical level coming out of the TV. Even then, you'd need the TV to have a feature where it switches off the internal speakers and then displays the optical level instead.
I know certain TVs have had variable Optical out in the past, but whether any new sets have it I don't know. Besides, would you really want to change the TV for this?
There is an alternative. The Sonos app runs on Android as well as Apple. It shows the real time volume level. There are inexpensive 7" tablets with Wi-Fi that can run the app. If your wife had access to a tablet she could see the volume level for herself.
It's also worth mentioning that TV ads generally play louder than programmes. You'll also find that some TV channels are louder than others. Those are good mitigating arguments for the defence, me'lud 😉 lol
On a serious note though, if you're finding that TV needs to be turned up louder then it's worthwhile getting a hearing check. We all lose some top end as we age. However, some of us also lose a bit of general sensitivity across all frequencies. If dialogue seems a bit muffled then maybe think about getting a hearing test.
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