Won't work if pin 4 of left opamp is grounded...

Down load a data sheet of the LM386; yours is missing some connections

Originally Posted by holmeschang

which part of circuit can step up the volume?

For starters,R5 in the right hand circuit adjusts the volume level, but you need to sort out the feedback components that are missing from the LM 386

Your microphones are not connected correctly.
Your opamps are inverting which makes their input impedance too low. They should be non-inverting then they will have a high input impedance.
The LM386 audio amplifier is missing important parts.

Originally Posted by Chippie

you need to sort out the feedback components that are missing from the LM 386

The LM386 is a little power amplifier (not an opamp) that has feedback components and biasing built-in.

Originally Posted by audioguru

The LM386 is a little power amplifier (not an opamp) that has feedback components and biasing built-in.

yeah my mistake....You are right as usual...

=holmeschang;807354inverting opamps? then how can I make it non-inverting? I had successfully run the circuit on the bread board but the outpt somehow very low...I mean the volume is very low...

Of course the output is low. The low input impedance of your inverting opamps are shorting the high impedance signals to ground.
You need to learn about basic opamp circuits to see the difference of inverting and non-inverting circuits.

About the LM386, I look thru the datasheet for missed component... it said that connect a resistor and capacitor from pin 1 to pi 8 can varied the Gain, what does it mean?

The gain is 20 but can be increased to 200 if a capacitor is connected between pin 1 and pin 8. A resistor in series with the capacitor will allow the gain to be selected from 20 to 200.
But your circuit is missing the extremely important 0.05uF capacitor in series with a 10 ohms resistor from the output to ground to prevent high frequency oscillation.

Pin 7 needed to connect a capacitor to ground, what is BYPASS mean? How it help my circuit? What happen if I did not connect them?

The bypass capacitor at pin 7 is used to reduce hum if a cheap power supply is used. If the amplifier does not produce hum then the capacitor is not needed.

Every article and tutorial about opamps show the difference of an inverting circuit with a low input impedance and a non-inverting circuit with a high input impedance.

The gain of the transmitting opamp is calculated by the value of R2 divided by the value of R1 plus the impedance of the mic which is about 3k ohms (if the electret mic has two wires). But the value of R1 is too low so it loads down the output of the mic.
If the value of R1 is higher then the gain is reduced. The inverting opamp with the low input impedance should be a non-inverting amplifier with a high input impedance.

R5 in the receiver is its volume control. if it is set too high then the LM386 little power amplifier clips the signal because its max output is only 0.45W when it has a 9V supply. Clipping causes severe distortion.