I was having the same problem...i was even regretting buying the phone because i cant even go out without seeing like 3 missed calls because the sound is so low!
anyway i am not rooted yet so this is how i did it
go to engineer mode by going to your number dialer and entering the following code without quotes
"*#*#3646633#*#*"
Be sure to enter the * and the # correctly.
if you have entered it right then the phone will automaticaly take you to engineer mode.
the fourth option or so will be "Audio"
click on it and then select Loudspeaker mode
at the bottom you should see something like value is 0-140 and below that Max vol. 0-160
change the max volume from whatever it is to 145 or below. you can go to 150 but the sound quality is bad and even if it sounds good dont do it cause it might damage the speaker. Im doing 145 and its perfect.
once you have decided on the number click set and then try calling your phone up
hope that helped.