Why can I hear music loud and clear on my mobile phone but I can't hear people talking through the speakers?

I have a samsung D500 mobile phone. I can play mp3 music off my phone without any problems but when it comes to people calling me, I can't hear them but they can hear me. What's wrong? I don't want to spend $$$ on another phoneWhy can I hear music loud and clear on my mobile phone but I can't hear people talking through the speakers?
You probably need to find the command that turns up the receiver volume. It took me a few times of not being able to hear people to realize that was what was wrong.

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