Volume doesn't work on remote of iphone or watch

Re: Apple TV: Volume doesn't work on remote of iphone or watch

Wondering if Apple support's online articles can help you with the volume control:

What to do: If the volume buttons on your Apple TV remote stop working - Apple Support

and

How to: Control Apple TV with iOS or iPadOS Control Center - Apple Support

Volume control through the physical Siri Remote works for most users by sending IR signals to the TV or audio system directly (not via the Apple TV box). As iPhone doesn’t have the capability to send IR signals, volume control isn’t supported in the same way there.

Volume control using the software remote on iPhone or iPad is supported for a few exceptions:

• Select AV receivers that support volume control through CEC. Not all receivers that support CEC would also support volume control over CEC.
• Stand-alone AirPlay speakers (e.g. HomePods).
• Stand-alone Bluetooth speakers.

Notably not TV speakers.

As mentioned in Control Apple TV with iOS or iPadOS Control Center - Apple Support

Note: This [volume control] works only with supported audio/video receivers.

The slashed speaker icon in your image represents the Mute function. The slash is not related to the availability. It is dimmed to show that this function is currently not available. For supported setups, Volume Up and Volume Down would be done using the volume buttons on the side of the iPhone.