Jessa is correct. If you provide more information, it'll be easier for someone to help you diagnose this issue. Based on the totally vague title and description of your problem, I'll take a wildly blind shot in the dark and say you are probably referring to the earpiece being hard to hear. Take a Qtip and rubbing alcohol and gently clean the mesh. Windex and a Qtip will often work also. The mesh will get plugged with debris/body oil/ etc and cause a "hard to hear" situation. Just a guess...

FIrst of all, iso alcohol ONLY helps absorb moisture, it will not resolve issues resulting from corrosion, etc caused by chemicals/minerals in the liquid the phone was in. You most likely have a short from corrosion under/around a component. The first step after verifying the phone is not starting and not recognized by itunes is to clean the logic board in an ultrasonic cleaner using a mix of distilled water and solvent like “Branson EC”. The shields should be removed from the board to ensure the cleaner can properly reach all components with the solution. Once the cycle is done, you can use compressed air to remove excess moisture and then soak the board in alcohol to remove all traces of water/solution. Replace the shields and retest. If there are corroded or shorted components, you’ll never get the phone to function properly or pass any tests and will only get frustrated trying. I know its not the easiest step, but it will ensure you aren’t pointlessly replacing parts in an attempt to get it going

This is a classic case of touch disease. The correct PERMANENT fix is to have Meson (touch IC chip) removed and a jumper put in place from the M1 pad to the corresponding circuit trace/pad. The issue is the M1 pad gets a micro tear/crack/break, etc and makes intermittent contact with the adjacent trace. The chip itself is (in my opinion) not to blame at all, but rather this defect in the PCB itself. You can normally reball Meson and put back in after putting in the trace jumper. A quick Youtube search for touch disease will have results including the M1 jumper mod. If you are unable to do the repair yourself, find a reputable shop that offers the service and send it to them. Hope this helps.

Is there any other damage to the phone? Was there any repairs prior to the update?

The iPhone 6 had a signature fault with severed/broken traces under/near the broadband CPU. It would cause issues like “searching for service” and GPS issues. I would recommend taking it into a 3rd Party repair shop that specializes in microsoldering. Even if it does not require microsoldering, the techs that do it will know how to test components and verify if it is this issue or not. This is , of course, just a guess based on the amount of phones displaying this issue that I’ve seen. Without seeing the phone in person, its impossible to diagnose. Good Luck!

It may just be in the app you are using. There is an option when setting up a new iPhone to “Import from Android”. You may try that (it’ll walk you through it), or try one of the many other free apps from the App Store and Play Store. Good Luck!

It’s also worth looking into replacing the battery. Due to recent updates, iPhones with older batteries (high number of charge counts) begin to suffer performance issues and can be very buggy. I’ve seen battery replacements resolve quite a few issues in the 6s and 6s plus.

Make sure the charging cable you are using is certified "Apple". Many people don't realize there is a control circuit built into the lightning connector in the housing of the cable. Certified cable manufacturures use the circuit board supplied by Apple in the manufacturing of there cables. Cheap knockoff cable manufacturers use a few cleverly placed chips in there cables to trick the phone into accepting the charging voltage. These cheap cables can cause all kinds of issues, even damaging the logic board badly enough to disable the phone entirely. If unsure about your cable, either purchase an original Apple lightning cable or use an approved aftermarket cable brand like ANKER (Amazon or Anker.com). Hopefully this info is helpful.

The components are not compatible between those models. I recommend a new screen and battery as first steps.

The LCD is where I would start. If you haven't opened up one of these mini's before then be sure to look through the guides on here for tips and locations of the sensitive components directly under the screen edge. Once you have it open and are ready to unplug the connectors, make sure you unplug the battery first. This will eliminate the possibility of blowing the backlight on the iPad. These mini's are infamous for backlight issues.