I've been struggling with the same problem with my Sprint S4. Some blogs say to blow out the link in the speaker area.. which works for some people. Some do phone reboots with or without battery removal... that works for some. Some have gone through trouble to do a factory hardware reset and that works for some, and not others.
Except for the hardware reset, I've done all of that and nothing worked.. Finally found what worked for me and doesn't require a factory reset. Go into settings, more..., application manager. Scroll to the right until you reach the "All" column. Scroll down to the "Phone" app, (not phone info) and press clear cache and clear data. Restart the phone (not just blank the screen, but reboot)... this should do it.