So grateful to find this thread. I bought an iPhone 7 privately off a classified ads site, in good faith. It looked to be in good condition, but it was reset, so I couldn’t test it. Then I discovered terrible sound quality on both speakers. Tried all settings, contacted Apple support, replaced the bottom speaker (the heat damaged the screen in the process). So frustrating, and no solution. Apple suggested to restore the phone and set up as anew device - still no result. Finally, after hours on the phone to apple, I saw this thread and looked carefully at the speaker vent - it was full of something. I cleared it out with a cocktail stick and it made a vast improvement. Then I realised that the mess I had found inside, with melted sealant and clear silicon all over the edges, was connected. It seems likely that a sloppy technician has used clear silicon to reseal the screen, but then wiped the excess all over the phone. The result is clear silicon sealant in the mesh/vent in front of both speakers. Various sites say that NOTHING dissolves silicon sealant, and the mesh is so fine that it’s impossible to pick out the sealant in the holes. So, a replacement of the mesh is the only solution. I found the bits on iparts-4u.co.uk for a few ££s. So now I have the screen off again and have removed the plugged-up parts with a little help from iFixit videos, a head torch, strong glasses and tools I got with replacement batteries. I am so grateful for all the info put out there to assist the unfortunate like myself. I only wish the technician who caused the issue had used these resources before being so careless. This phone was about to be a write-off. Now it will be totally fixed, costing me under £50 in parts. The screen damage was caused by using a heat lamp, designed to ease muscle pain, to soften the sealant. I learned my lesson and now use a travel hairdryer, using my fingers to tell when its hot enough - it takes surprising little time and makes it a lot easier. Thank you, thank you, thank you!