reduced mic sensitivity

The caller cannot hear my voice properly, or rather they say its feeble. I noticed whatsapp video calls were good, but when it came to send voice messages it was the same case. I tried a voice recorder app and noticed the voice recorded was really good when the top microphone was used, and it was feeble for the bottom mic. Is this a software issue or a hardware one? I contacted CAT and they told to reset it and see. Any suggestions? 

P.S - The phone was never wet