What's the difference between using 3.7v and 9v as phantom for electret mic?
The electret phantom power only drives the Idss of a JFET with a crystal ceramic mic. such that the gain and DC rise with the pullup resistor.  So it is ~ 1mA.  The expected max input of the PAM8403 on  5V at max gain of 24dB (~21) supply is +/-0.5V  which you could get by shouting into it.  So it is not the &quot;Line Levels&quot; expected from PAM8403 and needs a 10x gain AC coupled.
Pro Audio Phantom power is another beast with moving-coil and amplifier for high quality. But some run on 3V and 1mA like Sony.
Take these mics, which all use 48 volts. A Sennheiser MKH800 draws 3.0 mA, a Neumann U87 draws .8 mA, while a Shure KSM 32 draws 4.6 mA and all Earthworks models draw 10 mA.