Microphone and sound issues on my IPhone 7plus

Hi, as of now, I am unable to use siri, record videos with audio, use Voice memos, make facetime calls and hear/be heard on audio calls. Somehow, I can still listen to music and watch youtube videos. I've shut down and restarted my phone numerous times, but that has proven ineffective so far. I saw a youtube video saying it could be a faulty microphone, internal damage or another type of problem. Here are my questions:

1. What could have caused this problem?
2. Why can I still hear music and audio from apps like youtube?
3. Can this problem be fixed?
4. Is there anyway to fix this at home?
5. How much would it cost to have it fixed? (Might have wait a while because of the Coronavirus outbreak)

Thanks!

-Zach A