Sound not working on iPhone apps, but still working on videos & music?

A while ago, I had my iPhone screen cracked and got it fixed by a local repairman. Ever since then, however (about 3 or 4 weeks ago), the sound has not been working right.

Not one of my apps has the sound working without headphones. No matter what I tried - turning up the phone's volume to max, turning the app's volume to max, turning up the "Music" volume, etc.) it doesn't work unless I have my headphones in. If I have the headphones in, it works fine, but without them, it doesn't work at all.

That's only on apps. All my music DOES have working sound, as does everything online and any movies I rent.

Can anyone figure out why
(A) Only the apps have no sound; all music, videos, etc. Do have sound
(B) Why it only works with headphones in
(C) (most important of all) How I can fix this?

Flip the switch on the side of your iPhone near the volume control

There's a mute button on the top left side of your phone if it's muted it does exactly what you describe. Flip it back it should fix your problem.