I hate the idea of my kids riding around with ear buds in and not being able to hear a coming car, train, or my screams from the front door. But, 
as you well know, ‘I totally need to listen to my music, Mom!’. Meet your happy medium! Jam-Lite Essential weighs less than 1/2 pound and includes an adjustable quick release mount for your bicycle stem.
$39.95
https://www.audiblerush.com/bicycle-bike-speaker#.VELq7fldWSp