Voice memo/recorder microphone very quiet upon playback

It is far more likely that the barrier in front of secondary microphone at the top of the phone has filled with dirt and oil. This microphone sits next to the earpiece speaker, so often the two will become muffled at roughly the same time.

Sometimes this issue can be resolved by cleaning the outside speaker grill with a sharp pick and a toothbrush. However, I usually find it must be cleaned from the inside to be effective.

This would involve removing the screen first. Then you will need to unscrew the front camera flex and earpiece speaker. With the aid of heat, the front camera flex can be peeled away without damaging the several components on it. I would then use some heat and fine tweezers to pull the water proofing layers and rubber earpiece guide away from the glass. You can then use a pick and toothbrish to break up and push the debris out out the phone back though the earpiece mesh. I also prefer to put a drop of powerful solvent on the earpiece mesh and blast through oily gunk with compresses air. Care.must be taken not to allow liquid near the LCD.

In reassembly the phone, you should replace the glue seal between the screen assembly and housing with a new one