I did this and it now works just like it should...Maybe this will help you 🙂 Regards!
I recently purchased an iPhone 6 and immediately noticed that the phone call volume through the earpiece was much lower than my iPhone 5. Speaker volume was more than loud enough but the low earpiece volume (even though it was at max volume during the phone call) was making it difficult to understand anything.
A quick Google search for iphone 6 earpiece low volume brings up similar questions.
As mentioned by Ledsteplin on the iMore forums, a possible fix is to go to Settings > General > Accessibilty > Hearing Aids and toggle Hearing Aid Mode to On. When I did this and proceeded to make a phone call the volume was only at half even though I remember setting the volume to max during the previous call. I increased the volume to max and noticed a significant difference.
I noticed that when you toggle Hearing Aid Mode to On you can no longer toggle Phone Noise Cancellation to On (which is set toOn by default in iOS 8). I have a feeling the Phone Noise Cancellation setting is the actual culprit and the real fix is to simply toggle itOff, make a phone call, and set your phone call volume to max.