I've searched this community & the internet looking for solution for the past several weeks. After posting this question this morning, however, I finally found one possible solution.
https://itstillworks.com/make-iphone-microphone-louder-15836.html
INSTRUCTIONS: Microphone Volume Options
- Tap "Settings" and "Sounds" on your iPhone.
- Slide the "Change With Buttons" slider to the "On" position.
- Press the "+" button on the side of the iPhone to increase the overall system volume. Press the "-" button to lower the volume. This affects the volume of the microphone as well.
I followed the instructions and
- changed my sounds settings to "change with buttons"
- recorded a voice memo & turned the volume up completely (using the buttons) WHILE I was recording the memo
- tested & found the microphone recording was indeed louder
- changed my sounds settings back to NO "change with buttons" (as this is my preference) & retested - still good sound level on recording
Seems to have worked.