For example, when playing audio from a smartphone through car speakers, you can change the volume on both the phone and the car. It makes sense that when changing the volume on the audio controls in the car, the car’s audio system will send more electricity to the speakers to create louder sound. But how does changing the volume from the phone work? Is there some sort of metadata that the phone sends along with the data of the audio playing that tells the car’s audio system how loud to play it? In this situation, none of the electrical energy from the phone is being used to physically influence the car speakers to produce sound. How would the phone be able to influence how loud the music is playing?
So, there are two things at work here:
- The digitally encoded audio that the phone sends can just be scaled, leading to a louder (or less loud) reproduction by your receiving end
- Bluetooth's audio profiles do indeed specify commands to change the reproduced volume.
That's not "metadata" (that would imply synchronization with a specific point of audio); it's commands that are simply not part of the audio flow. Bluetooth is not a "single purpose link"; it can encapsulate multiple types of data.