Disclaimer: It worked for me. If it ruins your phone in any way. I will not be responsible 😛
It’s a common problem I see on the internet. Lot of people having problems with their Speakers after about half year or so. I did too. The problem usually is not that your speakers have lost their capacity to play loud anymore. Not that there is some hardware issue with it. Its usually plain and simple. There’s something blocking your speakers (ever laid your phone down with the speakers blocked?) and this in this case its dust. This particular phone has very tiny holes it its speaker outlet that its very easy for dust to accumulate and block it. What I had to do was remove this from those tiny holes.
One way to do that would be to remove the back cover and dust it off. But that’s kind of a long task which involves screws and screwdrivers. I did not want to do that. What I did do was switch off my phone, sprayed some LCD cleaning liquid onto a brush with very thin bristles (actually the one that came with the cleaning liquid) and using that to just clean it out. All I had to do was try to force the bristles gently into those speaker outlet holes and the dust either just stuck to the bristles or came out. After I was done cleaning, I waited for a while for this liquid to dry, just in case. Then Viola! Good as new. I had about 60-70% difference in sound level. Made me wonder how I actually managed all this while!