read though the tips n tricks section = http://androidforums.com/samsung-galaxy-s2/335847-tips-n-tricks-thread.html
As posted by HILT_M =
""""""I found the incall volume to be a little low. When you max out the volume using the volume button it actually only goes to a default setting of 87 out of 100. If you would like to change the default settings here is what you need to do.
call this number *#*#197328640#*#*
This will open the service mode menu.
choose option  AUDIO
choose option  (VOICE CALL)
choose option  HANDSET
choose option  Volume
choose option  SRC Speech RX Volume
choose option  5_lvl : 87
press the menu button (left side)
choose Key Input
enter a value between 0 and 100, I choose 100 the loudest setting.
now press the menu button and select back this should save (don't use the right side back key you normally use to navigate the phone or you will quit out of the settings) keep hitting the menu and back until you leave the service mode menu.
You will also note you can change volume settings for other things like bluetooth, head sets, video call etc.
Note: this is not permanent, if you turn off the phone it resets back to the default levels . """""""