Identify the access points in the pockets. These look like button holes with contrasting stitching.
Identify the access points in the collar. They are in the internal collar seam and also have contrasting stitching.
To wire your headphones, place your earbuds in one hand and insert them into the external handwarmer pocket. You will then reach all the way to the collar, feeding the buds out of the access point in the collar seam.
After your earbuds are through the access points, you can then wire the auxiliary end through the access point to the pocket where you've stowed your electronics.