I have sucessfully solved the problem in two moto g's
Hello to everyone.
First of all, I'd like to thank to everybody that have been developing the threads that as made this possible.
I have read many pages trying to understand the problem.
But I'm going to explain the whole process. I HAVE DONE THIS WITH SINGLE CHIP MOTO G 16GB AND MOTO G 8GB BOTH GSM!!!!
You can find the problem starting the call in a quiet place and you'll see that the sound is good, then in the same call you put some music playing around you and you'll start to have the issue. Try it and you'll find that is what happens.
I've seen mostly on Ebay, that this earpieces are selling like cakes, but definetly you don't have a broken earpiece, and you can test it activating SETTINGS-» DEVELOPER OPTIONS-» USB DEBUGGING, then you go to the dialer an dial this code *#*#2486#*#* or you may see in app drawer an app called CQAtest, it's the same thing, but this is only possible with USB DEBUGGING ENABLED.
Now that you have entered in the test menu you see two bars, select the first one, then select AUDIO, after that you see EARPIECE, click on it and make the BUZZ TEST, when it starts you should put your phone next to your ear. You'll be listening an harmonic sound that starts on a lower frequency and goes to an higher frequency, and it wil be repeating itself.
In this step you can for sure know if you earpiece has problems, if you don't listen any distortion in the loop that it's played, THEN YOU HAVE A FULL OPERATING EARPIECE. Otherwise, it's faulty and you should replace it ( I REALLY DOUBT THAT YOU HAVE A FAULTY EARPIECE).
OK. Now that we have tested the earpiece and we found that has nothing wrong with it, we have to take another steps.
THIS INVOLVE FLASHING THE WHOLE FIRMWARE TO JB 4.3 BUT IT WILL WORK REALLY BEATIFULL
I've taken the Android 4.3-Blur_Version.14.91.11.falcon_umts.Retail.en.DE
that is the one that really solve the problem, from sbf.droid-developers.org.
Both Moto G's were UK retail one with 4.3 first version (never updated, that so it couldn't install apk's unless they were downloaded from Playstore or GDrive) and the other one with 4.4.2, this can tell us that it is not a problem caused from updating firmware.
I really think even Motorola isn't aware of this problem caused by a little piece of software on the phone.
BEFORE YOU FLASH THIS FIRMWARE, YOU CAN TRY IF IT SOLVES THE PROBLEM GOING RIGHT TO THE SPOT.
Go to SETTINGS--» SOUND--» AUDIO EFFECTS.
Now you'll see a switch that can be turned to OFF, you can try this at first, go back all the way until the app drawer, search your dialer and make a call, could be to your voicemail or another service from your operator that you're sure you aren't paying. Iniciating the call in a quiet environment you can listen well but if you put some music to play from a radio or something that it is near from you, that will increase the surrounding sound and makes the problem reapears, if the switching OFF AUDIO EFFECTS doesn't solved the problem. Try this several times to make sure that the problem it is still there.
If the problem persists go to SETTINGS--» SOUND--» AUDIO EFFECTS and you'll see WIRED STEREO and SPEAKER. In the Speaker badge set Equalizer to OFF and repeat the process of the call in quiet and during the call add surround sound with a level that can match the volume of the call. Then again, if the problem resides in there it will reveal himself right in that point. Try this several times to make sure that the problem it is still there.
If the problem it is there after this whole process and tests, then to get rid of it FOR SURE, reflash or downgrade to the firmware I've mentioned before. After reflash, Go to SETTINGS--» SOUND--» AUDIO EFFECTS, select the SPEAKER badge and turn the equalizer to OFF.
Make the tests I've mentioned before in call quiet, and with surrounding music and you'll be pleased that THE PROBLEM HAS GONE FOREVER.
Hope I've helped.