The soution is easy but not obvious. Users Germantown and Boumbaier have it right above. Hopefully the following can clarify a bit.
With IOS7, Apple has moved the icon that is used to select output destinations for sound. This is the icon that looks like a rectangle with a small triangle at its base. It used to be that this icon appeared on the screen that showed what tune is playing in "Music" (and in other places within other apps). Used to be you would tap it and the choices for speakers and other output destinations like an airport express would appear.
This icon is no longer on the playback screen or in the Music app at all. It is now available at the system or IOS7 level, and is accessed by sliding open from bottom to top the new IOS7 window that Apple calls the "control Center". This is the gray window that slides open vertically by drawing your finger from the bottom to the top of IOS7's opening page and most other pages as well, including Music's "Now Playing page". On the opening page of IOS7, there's a visible handle at the bottom that you can slide upwards. On the "Now Playing page", there's no visible handle, but it still slides if you go through the motion, whereupon you'll see the rectangle with the traingle, waiting to be tapped to show the sound selections mentioned above.