Depends, if it has never been taken apart there should be little to no corrosion, the OLED issue is probably the phone hitting the bottom. Please note only get soft-OLED as they are the OLED type apple uses. Repair guide
Can you feel any vibration, if no the update may have corrupt the operating system. Before you try rolling back the update long press the power button for about 20seconds. If this does not fix it you will need to contact Xioami support
This is likely caused by an amplifier failure, first try a power cycle, unplug the power plug for 1 minute and plug back in, if the error comes up again take it to either a Bose service centre or a repair shop that does these kind of devices.
Contact the supplier and see if you can get a replacement, if that still fails ask for your money back and get it from a different retailer. If this still fails you may have a unlikely issue with the battery circuit