Hi Guys
I am an iPhone 6 owner, I was also facing the same problem as you guys. I don't think it is a hardware issue per se. Below is what I did and I think is a fix for this problem.
The new iPhones use VOLTE(Voice over LTE) technology. This means that you can browse the internet and talk at the same time on 4G. This technology actually sends the voice as data and not the normal call that you make.
Coming to the fix:
1) Go to Settings -> Cellular -> Enable 4G (Switch on cellular data if you can't see enable 4G)
2) You will get 3 options there: Off, Voice&Data, Data Only
3) Check which option is enabled for you.
4) If it is Voice&Data change it to 'Data Only' or 'Off'.
Changing to 'Data only' will route all your data through 4G speed, but the voice calls will be made through 3G. You will actually see 3G, while making a call. You can verify this.
Changing to 'Off', will switch off 4G completely. Voice and Data go through 3G.
I found that changing to 'Data only' or 'Off' improved and increased the earpiece volume on my iPhone.
This worked for me, hopefully it will work for you guys too.
I think sending Voice over LTE improves voice quality but decreases the overall earpiece volume.