Q: Phone 6 Plus low front speaker volume

Just to cover all bases... you did peel the protective film off the front of the phone, did you? It covers the headpiece speaker.

Sorry to say but sounds like a hardware issue to me. The Apple Store can test it for free.

Can you reproduce this issue when listening to your voicemails through the receiver as well? Also, is there any dust or debris that might have collected in your speaker for some reason in the last few weeks?

If you can reproduce the issue when listening to voicemail and there is nothing blocking the receiver, it's more than likely going to be hardware. You could try a fresh restore of the OS and test it out without installing any backups, but it sounds like it will be the speaker itself at that point.

Okay then yes that's going to be a matter of a hardware replacement for the receiver. They won't have to replace the full unit for that. I suggest making an appointment at your local Apple Retail Store if that is an option.

Contact Apple for support and service - Apple Support