Making a call will generally mute the music.
If you can set your music app to not mute (generally not considered a good feature) it will be playing through the speaker, not transmitting it to the caller/callee. Of course the other person might hear some of it as background noise, but it wouldn't be high quality (though it probably wouldn't be anyway: a voice call will probably have more compression than the music to start with).
In short, I don't know a way and if you could the quality would be crap.