Take a toothbrush, qtip, or toothpick and rubbing alcohol. Clean the top metal silver piece get the dirt out of all the holes. There is also a speaker on the bottom of the phone as well. Do not use water. Alcohol pads and toothpick or even a metal bread tie work. 

If you scroll down at the top of your phone, next to where it has the brightness there is an auto button, hit that and it makes it louder bit it will eventually go low again then hit the auto button again. It works but its annoying

Hi Breeze! Try using 3rd party apps like Ultimate Volume Booster (link). Hope that helps!