When I ride my bike solo, I like to listen to podcasts and music in my right ear. I leave my left ear open to hear the other sounds around me. You can try telling me this is a bad idea or illegal, but you're wrong. It rocks.
I run the earbud cable down inside my jersey, and in the past cut a small hole inside the rear pocket to pass the plug through to plug into my phone, which I keep in a jersey pocket.
I've wanted a better way to make the hole and experimented a bit with grommets to see what works. Here's how I hacked my newest jersey.
You can pick up a grommet kit at a fabric store like JoAnn's. It's not that expensive, under $10. Make sure you get the kit that includes the mini anvil and setter.
I place the grommet in my right rear jersey pocket. Since I'm right handed, it's the easiest pocket to reach into and pull the cord through.
When you are sure the setup is in the place you want, inside the pocket and only through one layer of cloth, you whack it with a hammer to set the grommet. You need to hit hard since you are bending metal. I used a nice, heavy framing hammer.
Pull out the setter and the anvil and you should have the grommet in place. You might have to trim an excess fabric.
I placed the smooth side of the grommet inside the jersey so it doesn't catch against my base layer or my skin.
Here's the grommet with the gigantic plug feed through. Your plug will probably be smaller, but I'm using the One Good Earbud which for some reason uses the giant plug.