I was looking at the post more, and I think I've finally realized what you're really trying to do
You're not using the aTV to watch videos, you're using airplay to connect the second monitor for your Mac. Since your existing cable died.
If you're doing that just use the volume controls on your Mac. don't worry about the aTV remote.
Just set the volume on the speakers at a resonable level and leave it alone.
You could also plug those speakers directly into the Mac. might make a bit more sense.
The audio normally goes along with airplay, but you can change it back
go into system preferences -> audio -> output, you can set volume and pick output from there.
or if you have the "show volume in menu bar" checked, you can adjust volume and choose output from the menu bar.
or if you have an apple keyboard, there are volume buttons on there.
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I had typed all of the below before I came to the realization above,
The ATV controls volume in 3 ways -
A: when audio is connected via bluetooth (headphone or speaker).
Airplay should be included in this too.
not sure if it needs a new letter, or just change it to "A: remote speaker, either bluetooth or airplay"
via ARC (telling the destination to change volume over the HDMI connection)
regular
HDMI signal contains audio
It's used for most connections. aTV to TV, Cable box to TV, aTV to receiver, and so on.
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HDMI-ARC (audio return channel) is where you run audio "backwards" down an HDMI cable compared what you would normally do.
Audio is coming
out one of the
inputs on your TV and going
in to the
output of your audio device.
if your TV is Plugged into the output of a receiver, and your cable box is plugged into and input on that receiver
When you're watching your cable box, you're only using HDMI, since the receiver is just grabbing the audio as the entire signal passes through on the way to the TV.
Now lets say your Receiver doesn't support 4K, so you plug your aTV directly into your TV.
The combined audio and video go to the TV by HDMI.
Now just the audio goes out to the receiver via the same cable that brings video/audio up from the cable box/receiver. That's HDMI-ARC
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HDMI-CEC (Consumer Electronics Control) is sending controls over HDMI. (think button presses)
Manufacturers use many different names for this, but for the most part they all work the same, and work together.
Some examples of names are SimpLink, Anynet+, and a number of others that normally including "sync" or "link" as part of the name
This is how you control the volume on external devices,
**The aTV can send a "Volume UP" command over the HDMI, depending on your setup, your TV could either do that, or pass it along to your receiver.
**your TV can send a "move left" or "Pause" command to the aTV or Blu-ray player
**When you turn off the aTV, it can send an "all off" command which will turn off both the TV and receiver.
