Using headphones with bone conduction.
I was using headphones with bone conduction while riding. You can hear clearly outside world and the music both. Your brain will automatically select the loudest sound from both two sources. Their problem that in a road with high car traffic there is an engine sounds that are louder then headphones, so, you dont listen to the music actually. After a year, i switched from headphones with bone conductuin to external speaker (one of JBL) and placed it on an internal frame traingle. After some time I changed it place to a side pocket of my backpack. In both places I had to make it macimum loud to hear. So I am now sure that sound system music source must be in front of you, as near as possuble to the ear to make it loud enough to hear and quite enough to othera not hear (wind blows off the sound whule you riding, so it is hard for sound to ger from a bottom of a frame to your ear while you moving). Next will be a little speaker placed on a handlebar so the sound will have mych shorter way to get to my ear, and i can make it mych quieter so other world wil not be annoyed by my music and i will not have to make it's sound volume too high to hear, and that will save batteries)