Why does my speakers distort?

@mojkraj it is possible that this is a dirty speaker mesh/grille. This will cause a decrease in volume more than anything else. Try to clean the speaker mesh/grille. Take a clean, dry, soft-bristled brush and gently brush across both the top earpiece slit and the bottom loudspeaker holes.

After that, you want to check your phones volume by connecting a pair of head or ear phones. If the volume is loud and clear through the headphones, your physical phone speaker is likely damaged. Now, if the headphones are also quiet then the issue could be related to software settings or it could be an issue with the audio circuit on the motherboard.

To replace the speaker and since iFixit does not yet have a specific guide for this, use something like this video At 9:20 you see the speaker getting removed. Of course, you do not need to remove the motherboard etc. It should be possible to remove the speaker right after you remove the midframe shown at 8:46. Replacement speakers are readily available at many online places.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same issues with their device. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!