How can I fix iPhone 5 volume issues?

My iPhone 5 has begun to have some issues with the volume. For example it will not recognise when headphones are plugged in or not. When I want to play music on the speakers (using ANY music app) there will be no sound playing at all.
However when I get a phone call or my alarm/timer goes off then the volume is fine and I can even lower or raise the volume.
The problem has been like that for a while now. Now its begun to affect my calls. Sometimes I cant hear the person who is calling and Im forced to talk to them over the speakers or headphones.

How can I fixed this problem? How do I know what caused it?