Hi, My HTC One's top speaker is blown. I have bought it from abroad, hence I don't have warranty in my country. Therefore...

Yeah, according to some internet search, the factory reset does not seem to have helped anybody. But it still may help you, who knows. If you give it for repair, they will factory reset it anyways. You can also try safe mode. It is very similar to a factory reset. No downloaded app runs. But it is temporary. After a reboot, you are all fine. Make sure "Fast boot" is not enabled, Shut down your phone, Open it, When you see the HTC logo, press and hold volume down button. It will open in safe mode. Not a factory reset but if an app causes this problem it will reveal it. The only difference is, the settings are saved and the only settings which are relevant would be volume itself and beats audio, so it shall not differ between a factory reset and safe mode. By the way, HTC One's speakers are loud, but not extremely as thought. For example, Note II and Nokia 920 probably have louder speakers. But HTC One's speakers are much more clear, and basses are so vibrant. So its speakers are not the loudest, but the...

Don't worry. Use it as is. One corner scratched, no problem at all. You should see mine. You may try to paint over the scratched regions with a permanent black marker or so. Also you can use a case, which would hide the present scratches. While trying to disassemble your phone, you may mess things up, and become even more unhappy.

Yeah. I had the exact same problem. I have inserted a few strands (like 2 - 4 etc.) of a toothbrush into the bottom mic hole very carefully and slightly rotated them inside, in order to wipe any dust accumulated inside this hole. And it works like a charm now. Be careful. Do not harm your mic while trying this. You may puncture it etc. I don't know the risks exactly, no guarantee. Try it at your own risk.