Volume control through the software remote on iPhone is supported for select AV receivers that support volume control via CEC (from the Apple TV box). When supported, the volume buttons on the side of the iPhone are used to relay this Volume Up/Down signal. The slashed speaker at the top would then be for the Mute function.
There are no known TVs that support volume control in the CEC signal from other devices.
The physical Siri Remote uses IR to send volume control signals directly to the TV or audio system. That is obviously not available for the software remote on iPhone.
You can use iPhone (with iOS 17) to help locate the Siri Remote (2nd generation or later) (tvOS 17; firmware 0x83).
If you lost your Apple TV remote - Apple Support