Why can't person on other side hear music playing in my phone during a call?

I think your are misinterpreting ambient sound

• You are in a crowded , noisy place and are talking to someone on phone. They can't hear you clearly because of the ambient noise around you. Why? Because the phone mic captures your voice and noise around you

In this case what you hear is what the other party hears = Ambient noise + your voice

• In the case of music playing on your phone (media player running) and you talking at the same time, it is not ambient . Phone mic picks up only your voice (while what you are hearing is both streams media and your own voice) because voice is prioritised cutting out media. Once you establish a voice call

From this point on the telephony service has the highest priority and won't let any other audio play in parallel.

In this case what you hear (media + voice) is not what the other party hears. He hears only your voice. Think of it as you can hear two channels or streams whereas other party can only hear one stream of your voice

Now, if you were to play the same media from your laptop speakers while you are talking that's the first case of Ambient noise

Related Direct Music Player audio to Caller