Apple - 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.

The solution you mention worked perfectly for me. I have an iPhone 6 and speaker volume was painfully low (I even thought I was getting deaf). Speakerphone calls were simply impossible to handle.

Instead of taking it to the Mac store, I used one of my camera's lens cleaning brush (NIKKON LENS PEN is $5 dollars at Amazon) and in less than 1 minute, sound was doubled to my estimation.

The benefit of having an Apple technician clean your device is if they break it (pushing in the bristles too far), they may have the ability to have the business repair the device at no cost to you. The down side of having an Apple technician is you have to go to them and travel and perhaps wait. If you're comfortable with a gentle toothbrush and won't press the gunk in deeper and make it worse, go ahead and try a DIY cleaning.

As long as the substance isn't oily or going to get compacted, you can clean your speaker grills quite effectively with light air or a mechanical duster.