Sounds like something is shorting causing everything to ground out making the connection to them live. It would explain everything going on like the horn, lights and more as the current could pass through them completing there circuit and making them go off. I'd look for any exposed wires and things like that and of you're only getting 4v the rest of that voltage has to be going somewhere. Also check if the battery is still giving the required voltage out and if it is then follow the leads and look for any thing out of place from the origin to the end and work from there.

Always disconnect the battery when working on electronics. If you drop something metal on the board then it's shorted somewhere, if a connector doesn't go in right it's shorted, if you tap something anywhere that's an exposed contact you risk shorting it. No matter how small you should always disconnect the battery and unplug the device you are working on to reduce the risk of damage.

Of it's Bluetooth connected then a poor signal could cause this. If wired then a poor cable (damaged or the tip is covered is crap) could cause it and also a headphone jack on your phone or PC could be filled with dust and crap that builds static causing it. If it's something that always happened from day one then idk but if it recently started the above could be a good answer for you.

If you replaced it ( you opened it up) open it again and check the connections you messed with it could be the battery isn't fully connected as you should disconnect during a repair to avoid damage from power still being in the board. If someone else replaced it (paid a shop or company) take it back and have them fix it. It should be free for them to fix it as they broke that part of the phone or failed to repair it properly and should honor they're work.

It's lost a lot of life if not stored correctly. You can try to use it but it could be bad and since it was never charged it's completely discharged and could be damaged meaning a risk of fire or battery expansion.

Go to settings and go to Battery or battery usage and see what is using it all also location services drain battery's quickly or you could have a faulty battery which can be replaced

Try turning it on and off again and then use Aida 64 to see if it's mic is even being found

Most likely the connection between the monitor and mother board is old and dead or damaged in some way

You need to flash the firm ware on it because the one on it now is damaged and cannot self repair like it was trying to in the boot loop. I don't know how to flash a firm ware because I have never needed to but you can find tutorials online

I am 90% sure you damaged the ribbon cable or the connection housing in the odds is the ribbon replace the home button and it should be fine if not then the connection between the ribbon housing on the main board and the board is damaged and you are up a creek with out a paddle