I've seen someone with small mirrors that replace the caps at the end of the drop part of the handlebars. They're about 2 X 1 ", ellipse shape, slated. She really liked it. (Sorry, couldn't find any on the web to show you.)

Other option is one of those small mirrors that clip on to your glasses/sunglasses. A friend of mine has worn his forever and can't live without it.

I however use none. I wish I could listen to music while I ride, but no. I try to remain extremely aware of the sounds around me and have practiced a lot at turning around to glance at my angles without swerving. That's an important skill I think. Even when there's no noise at all I glance back once in a while as there could be another bike behind me and that could become a security issue too (plus I'd hate to blow my nose or spit and then realize there's someone behind me!!! haha).