i don' think an IR code exists for this.  you can do this with RS232 using the exlink jack.

What i do in a case like this, is put a secondary set of volume commands on the remote, usually on page 2 of the activity that is in use.  or if the control system allows, I put a toggle button on it and have the volume buttons toggle between the TV and the AVR.

depending on the AVR, there may be an IR code to enable or disable HDMI passthrough.  turn the passthrough on, and you get volume from the TV speakers.  turn the passthrough off, and no volume from TV speakers.