I don't feel like i want to change the original drivers of my phone.
If it's the case to change the audio drivers I will replace them only with an updated original set of drivers from Sony Ericsson.
For the moment I don't think it's a question of drivers, because the sound it's pretty good when the phone rings. Only starup sound and ringtones preview mode has a low sound level. That's the reason I asked if it's a software preset that can be changed to enable a normal sound level at startup and in preview mode.