How can I fix a low speaker volume problem on my iPhone 6s?
Answering my own question.  

Got it fixed. In case someone else has this issue, I took it to the Apple store. Turns out the speaker got clogged with microscopic particles (lint, sand, etc...) since I spend a lot of time outdoors. They have a very fine brush that can get those things out. The whole process took like 5 minutes.