Q: After upgrading iPhone 6 to 9.3.1, my to-ear speaker volume is too low

i have  iPhone 6 when i upgrade ios from 9.2.1 to 9.3.1 , the normal speaker volume is super low.

Restored back to 9.3 and still having problem.

Any ideas?

Anyone have a link to a signed copy of the 9.2.1 ipsw file?.

plz help to solve this problem.asap