Earpiece and loudspeaker issues despite replacements

I have an iPhone 6 which in recent months has begun showing signs of trouble with its earpiece and loudspeaker:

• When on a phone call, the volume coming from the earpiece is very low. It is almost impossible for me to have a conversation with somebody if I am outdoors (especially in the city or busy shopping areas, etc.) because the ambient/surrounding noise makes it very difficult for me to hear the other person.
• If I place a call on loudspeaker I can hear the person on the other end of the line but they cannot hear me at all, even if I place the microphone right in front of my mouth and speak loudly. There are no problems with the microphone when I use the phone regularly (held up to my ear) though.

I assumed both of these problems would be hardware related so I ordered a replacement earpiece and loudspeaker. I was also having trouble with my lightning port so I also ordered a new lighting assembly, which includes the microphone. I installed each of these today, and in the case of the earpiece and speaker this included a vigorous cleaning of the speaker grills (there was a lot of built-up gunk and dust both inside and out which was removed).

My lighting connector now works perfectly and the volume/quality of the loudspeaker during calls or when listening to music is much better - but the earpiece volume is still very low and my callers still can't hear me when using loudspeaker during a call. So it seems the issue may not be related to hardware - or at least not the actual earpiece speaker, loudspeaker or microphone. What else could explain the cause?