Fran Montero is right.
If you go to the MTK engineering settings, you're able to tune the volume amplitude for each audio output and for each audio level.
For this, access the MTK engineering by using a specific program in the playstore or by compsing the following code in the phone dialer: *#*#3646633#*#*
In the menu, go to the "Hardware Testing" tab, and select "Audio" in the list.
Next, select "Volume" , then "Audio Playback".
In this new menu, you have to choose which source you want to modify, and after which audio output you're targeting.
Below these 2 settings, you can see a list of 16 indexes (Index 0 to Index 15).
These indexes corresponds to the number of steps you have when you press the Volume +/- keys.
In you're case, you should select "Music" as source and "Speaker" as audio output.
If you want to level down the sound of the lowest audible level of your phone, go to Index 1 and change the value to a smaller one (Mine is set to 88. it could be set to 50 for example).
After you modify it, go completely down of the screen, press the "Set" button, wait for the successful setting pop-up window, confirm it, go out from the MTK Engineering menu and restart your phone to take your new audio value setting in account.
If everything's goes well (and if I'm not wrong with my advice...) your phone should output a sound level less louder than usual in the first volume level.
If you want to change all the audio steps to have a more lean volume curve, feel free to do it as you want.
Just keep in mind that each level can be set from 0 to 255 (0 = Silent / 255 = MAX) and that it's not a good idea to apply the maximum value in one of the settings, as it could harm the audio device output if continuously used at this max level.
To tell you the truth, it seems the default setting of the P9000 is not very clean as I can see that all the Index 15 audio output settings are at 255 value....
I can try to use an Oscilloscope to see if the sound is not distorted by these innapropriate value.
Or maybe the PGA settings has something to do with that. I haven't found the meaning of this setting, but it's probably the value setting of the Gain used for the power amplifier linked to each audio output.
Need to clarify it, but I don't have time now.
Tell us if these advice worked for you.