Q: How can I change the volume on my TV/AVR by using the Xbox One?
A: Once the Xbox One has been configured to control your TV and/or AVR, it can also perform actions such as mute and modify the volume. This can be done through voice, using the Xbox volume up/down and Xbox mute/unmute commands.
However, the volume up/down commands only make a small volume change, so to make a large change to volume would be a lot of Xbox volume up/down commands. Here's a full list of Xbox One features and voice commands.
A better option is to leverage the Xbox One SmartGlass application, which is available for most platforms including Windows Phone, iOS, and Android, and use it as a remote control.
After the SmartGlass is connected to an Xbox One (that has been configured to control TV/AVR), a volume icon is shown in the bottom right corner (see screen shot below). Press the volume icon to see plus, minus and mute icons to quickly change the volume.