It's not the volume button.
1. I'm sure you know to have a headset in your helmet.
2. Headset cord plugs into the bike. The bikes plug is on the left side, top of the fairing next to the glove box.
3. Push and hold the audio button until the screen shows it's on.
4. There will be a small icon show on the left side of the screen. If it looks like a speaker, the bikes speakers are on. If it looks like a headset, the sound will come through your headset.
5. Push and release the audio button quickly to toggle between bike speakers and headset.
6. If you want FM, push the FM button.
7. You can then use the tuning knob on the right to get a station and the volume knob on the left to adjust the volume.
That should get you started
There are others options for ambience and auto volume control, etc.
All these should be covered in your owners manual.