Is it iPhone 6 or is it the app?

I just bought the iPhone 6. I never drop it or anything. I don't over charge it or plug it in the charger and pull it out. I wait till it's dead to charge it. Anyways the volume I have a problem with it was fine the other day until yesterday snapchat stories will play but you won't hear the sound until you mess with the volume and I was playing 8ball and it's the same but the other apps sound work fine but just those two so far. Today I try my head phone and it work fine I didn't had to mess with the volume. Is it the iPhone 6 or is it the app?

TRY THE ISTYLUS