why does the macbook volume control not control volume on apple music via airplay

If the Music app is redirecting content to external speakers via AirPlay then the volume control in Music will control that. The Mac's own volume control will then only affect other sound that is routed to its speakers, e.g. content from a web page. If all audio from the Mac is being routed to AirPlay speakers then the Mac's volume control will have an effect.

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