The answer about the servo connections is in the Robot Tutorials found above (under the SoR page title).
How would you use one microphone amplifier circuit work for four independent microphones?
Also, I think the programming is going to be harder than the circuit itself.
If you are not experienced with assembler language programming on a PIC then yes, the programming will be hard.
It is best to break the program into smaller tasks and get each working. For example, get the code working to control one servo, then add code to control two servos.
Separately write code to to measure the amplitude of a microphone input, then two mics and a compare.
Then finally put all the code pieces together.
The ADC's on a 16F877 are not that fast at conversion and may not be able to digitize the AC waveform from the microphone so that the code can calculate the frequency. Instead you may need to create a circuit that produces a DC signal that is proportional to the frequency of buzzer heard by the microphones.
Another way is to ensure that the output from the microphone amplifier is large enough to provide clean digital logic levels. Then use the PORTB IOC to measure the period of each input (1/period = frequency).
This part of your project will probably be the most work. The servo control part should be relativity easy since many people have done this and there are a number of code samples on the web.