Electret microphone amplifier - Terrible low frequency response
Ratio R(collector)/R(emitter) roughly sets the gain. Without R6+C3 your circuit has very low gain, ~1.5. Make R5 say 300 ohms and put R6+C3 parallel to R4 and set R6 equal to R4. Adjust R1 to have 1/2 Vcc on collector.

The purpose of R6+C3 in the current circuit is to bypass AC and block DC; it necessary to keep circuit output reliably at desirable voltage - no matter what the gain you won't need to fiddle with bias resistors. You may try C3 1000 uF, may help.