Why is my Samsung S4 speakerphone behaving funny?

[AC Question]

Why is my samsung s4 speakerphone behaving funny

Here's a curve ball. My s4 behaves for some hours after I start/reset the phone. Then it doesn't ring on incoming calls, music doesn't play from speakers, no sound from YouTube. On the other hand, if I pick up a call and transfer to speaker, it works but the voice has an echoing effect.
The phone is not on mute, I have done factory reset, android version is 4.4.4. Do you think it's some software glitch? Or the motherboard behaves funny after working for a while?

 

[B. Diddy]

Re: Why is my samsung s4 speakerphone behaving funny

Welcome to Android Central! Try booting into Safe Mode, which temporarily disables all 3rd party apps. https://support.t-mobile.com/docs/DOC-11724

If the problem disappears in Safe Mode, then something you installed is causing the problem. You may have to uninstall apps one by one until the problem disappears. If the problem persists in Safe Mode, then it's more likely to be a hardware or firmware problem.