How to to control my IR receiver (volume) with the apple tv remote?

Verify the settings for Volume Control. The default is ‘Auto’ which should work for most users and equipment, but if it doesn’t, then you may change that to one of the other manual options, i.e. ‘Receiver via IR’ for use with an AV receiver.

If that still doesn’t work, then the audio system may need adjusted model-specific IR signals instead of the standard signals for volume control. To configure that, choose ‘Learn New Device…’, where the Apple TV can copy the signals from the audio system remote, to use with the Apple TV Remote.

The Siri Remote 2nd or 3rd generation shouldn’t matter. They have the same capabilities, except for the charging port.

Unless someone else chimes in with a new idea, I would suggest to contact an expert from the Apple Support staff for further assistance. Contact Apple for support and service (phone numbers) or start your support request online (chat/​email/​callback).