AbhinavDid you try to check after factory data reset? Ifnot please give it a try and see if that fixes speaker low volume issue.
Actually I don't remember how my phone's speaker was in Android pie or in any other older version of Android. But what I can definitely say that my phone's bottom speaker sounds lower than my friend's 5z's main bottom speaker.
Main speaker's sound output is manageable but I many times had complained to mods that the sound configuration is bad and not same as Like 5z had when it launched.
I had purchased my phone on 21st July 2018.
As of now I can't afford to change my phone's speaker. Will change it at later point. I'm waiting for the next OTA.
I didn't listen to music via phone speakers any frequently. I generally use the ZenEar pro which came inbox with the phone.
amitgarde3I already did that but still no success.
Did you try to check after factory data reset? Ifnot please give it a try and see if that fixes speaker low volume issue.