Under the Simcard tray you will find a sticker, try to keep this sticker because i think it has to do with insurance, there is no other stickers what i can find that voids guarantee.
The T5 in the earphone jack you can only reach with a really thin t5 torx, and it?s important that you do not force it because you can break alot of sensitive plastic inside, but no worry, the phone will work anyway, it will just not close up as nice afterwards if you brake something, like i did, twice.
I marked wich direction you unscrew it, but im not 100% about this, but try, check for dust first if the t5 doesnt fit. and also check your t5 screwdriver, i bought a cheap one and it did not have the right number of teeth and i ruined my screw a little bit.
Here you can see the screw that moves the whole locking mechanism inside.
it moves a slider up and down that holds some plastic hinges in place, these can break if you try to hard. you could also figure out the direction for unscrewing it by locking at this image, but im to stupid for that.
here is the slider, its actually quite smart, just a little bad plastic, not hard enough
And heres the hinges that connects to the slider, i broke two of these
when you unscrewed these two screws, the side of the earphone jack should pop out easily, but do not bend the two sides apart to much because the other side has push in holders, you need something thin, preferably a regular phone toolkit, and crowbar the screen right out from the back, no bending. there is several youtube videos for dissasembly lumia 928.
when you have opened the phone, the earjack module just pops out of its place if you put the screwdriver in the hole and bend it outwards a little bit, no real need to remove it, but its easyer to clean.
in the next image the arrows point out the section that does the soundchange, if you do not remove the module, and the phone is still running while you do this, wich i do not recommend.. You can put in a headseat and you will se wich part is moving towards another part, and if they touch, you should be able to change the EQ in the phone, usually there is just dust thats in between the metal plates that hinders the connection, but on my phone i had to bend it a little, but put in a headset a few times so you understand the mechanics.
in the next image i marked the metal part green, this you can pull out with a tweezer, but be carefull so you do not bend of the contact pin that sticks out.
This part meaby needs some bending to be closer or further from the part that moves when you put in a headsett. but be carefull so you do not bend it of. and sitt in a god workplace, i did this photoguide in the livingroom, stupid and what not, and i lost the part in the floormat and i cant for the life of me find it again, so i ordered a new module on ebay... but i will buy some thin metal and cut out a new peace and see if this works to.
sorry for the bad images.