With the phone awake and the home screen showing, tap the volume buttons on the side of the phone and see if the ringer information shows, or if it has a line through it and shows headphones, even without the headphones plugged in. If so, you may have to either try plugging and unplugging the headphones a couple of times, or check to see if there is an debris in the headphone port that is making the phone think headphones are plugged in.
What other troubleshooting have you done? Ringer volume and volume for other things such as the sound in the earpiece when on the phone are two different settings, as well as the setting for the volume of the Music player.