iphone 4s no sound from apps

Hi;

My Iphone 4s has stopped playing any sound that comes from apps (including music). I know the speakers are working, as normal system sounds are still fine (ringer, text tone, alarm sounds, siri). In the case of the alarm clock, the sound only works if I use one of the pre programmed sounds. My own music cannot be heard as an alarm ringer.

Any suggestions of a fix?

Thank you!