I'm having problems hearing phone calls clearly through the earpiece and loud speaker. However, old voicemail messages can be heard clearly through the earpiece and music plays back clearly through the speaker. Anyone any ideas what the issue is?

I'm recently having some problems hearing people clearly during phone calls through the earpiece on my iPhone 5. I tried switching to loudspeaker and I'm still having the same problem - the sound is all broken up. But when I play music through the speaker it's clear and when I playback old voicemail messages through the earpiece, they're playing back clear as well! Anyone any ideas what the hardware issue could be and if it could be easily fixed?