Question about Marmitek Connect AE14

No, it cannot! You can only adjust the volume with the volume control of the audio system. Even if you switch the television
on mute, the signal that comes to Marmitek via HDMI ARC remains constant (quite loud in my case - I have to lower it a lot with the volume control of the audio system, but that depends on the system). By the way, this way you can use the television as a "central speaker" - if it has excellent sound.
as a "central speaker" - if it has excellent sound!... - but it is also possible that there is a time delay of the sound, and then the whole sound could be "greased".
Personally, I am very happy with Marmitek, as it allows me to listen to SACDs digitally via my "high-end" DAC.