[Galaxy S 4 Q&A, Help & Troubleshooting] Yoshi_523:
Okay so I after trying around a dozen custom ROMS, scouring the internet for ages and pulling my phone to bits multiple times, I was finally able to solve the problems with my i9506 microphone not working during voice calls when the speaker phone function was on. Basically the person I would call could not hear anything at their end whenever I turned on speaker phone function, before I solved this. This guide should also help you to get the other mic working for the standard voice calls or the voice recorder, if that is your problem. My Sammy S4 i9506 has 3 mics. I think most of the Sammy S4 versions such as the i9500, i9505, ect have the same basic construction as here and include 3 microphones. 2 mics are at the top near the 3.5mm headphone/audio jack. They are secondary mics used mostly for speakerphone function (or if you enable stereo voice recording). On the outside of the phone you will see a pin sized hole inlet in the silver frame near the 3.5mm jack. The primary mic is down the bottom of the phone. On the outside of the phone you will see a pin sized hole inlet in the silver frame near the micro USB port. It's used mostly for voice calls (when you don't have speakphone function enabled) or for mono voice recording.

So first up here's a summary of the main fixes I found...

1) The rubber inside the case around the microphones hase really small holes for the voice to pass through and also a fine mesh layer like paper. This as well and the pin prick sized wholes in the frame easily can get blocked up with dust.

2) Surprisingly enough after trying over a dozen custom ROMS (and following their install and root procedures to a T), I found that most new ROMS based Android 6, 7 and 7.1 all had issues with the microhone getting muted when selecting speakerphone during a call (Also had other bugs like while playing music and connecting the 3.5mm jack). I had to go back to the latest Original Sammy ROM from SAMmobile to get a usable speakerphone option in a call. Currently using the SAMmobile stock Nordic ROM Android 5.01 form early 2017. Very sad as the RessurectionROMs & Linage ROMs so far I really like the most slickest Custom ROM on Android 6 and 7, and almost everything works, except a couple of base line essential functions like the ones mentioned that I can't live without. Very sad, indead and I believe this is bug exists probably becuase all the these ported ROMs are based on Cyanogen code and drivers. Anyway just thought I'd mention that even after lots of nice customisation on the Stock Sammy ROM like De-odexed, Slim Rom + Root + numerous Xposed visual improvements ect the speakerphone and everything works!

Okay enough jibber jabber. Lets get into the details of the fixing the inherant dust build up in the 3 microphones rubber peices.

You will need to follow all the dissasambly steps to remove the heaphone/3.5mm audio jack.

These can be followed from the guide here:

https://www.ifixit.com/Guide/Samsung+Galaxy+S4+Headphone+Jack+Assembly+Replacement/16539

At the pottom of my post you'll see my attached Pics to give you some instruction on the parts to clean...

Image 1 is the basic tools & cleaning kit I used. The main tools here for dissasembly are a small jeweler's size philips head screw driver and some plastic pry tools. (A guitar pick might be enough). The tools plus a couple more pry tools can be bought for around 2 bucks form ebay direct from China if you search for Samsung s4 dissasembly tools kit or glass replacemanet tools.

The fomulation of Alcohol may be different where you live. Basically here "Methylated Spirits" simply is 95% Ethanol plus a nasty bittering agent to stop injestion. The MSDS says it does not contain the highly toxic methanol so it should be safe to use without gloves & in indoors. Isopropanol (Iso-propyl alcohol) is more toxic so if you use it make sure it's in a well ventilated area. Methanol is just too toxic an alcohol for me. The risk of absorbing though the skin, breathing in the fumes ect and it causing blindness & liver failure are not worth it IMHO to use as a general cleaning chemical.

Okay... late night for now... I'll try post more pics & instructions for cleaing the bottom voice phone mic... but if you want to find it yourself it's another rubber rectangular peice in the frame located near the bottom close to the micro USB post. Again you'll need to stick a small brush with alcohol in it to clean out the thin tube/hole.