Equalizer is horrible broken...

[MartinK1]

Goto SOUND options in SETTINGS, 

SOUND EFFECTS -> SOUND ENHANCEMENTS -> pick any preset

Set all bars to the same value

ALL +10 -> should produce higher sound, instead the sound is hissing !!!

ALL 0 -> should be neutral, it is, its therefore OK

ALL -10 -> should produce lower sound, instead the sound is muddy !!!

what the heck !

I believe that an equalizer should be 'neutral' 

[Samuel1]

If you believe that there might be something wrong with the function in your phone you can repair the phone software via  PC Companion..

Before repairing your device you may want to backup your information first. Check out this topic for more information on how to.
How to backup?

If the issue should still remain I think that this needs to be examined and fixed at a repair center. For more information about how to submit your phone for repair and where, contact your local support team.

[MartinK1]

Reinstalling the phone won't help solving a broken feature on the phone