Electret microphones will only provide a very weak voltage signal so the breakout board would be good as it then amplifies it into a much larger voltage swing.
As for boards - then most microcontrollers provide at least one UART. You need to consider what port you will use at the PC end. If you have a RS232 serial port then you will need a level shifter to connect the UART to the RS232 port. Modern computers normally come with USB ports instead. In which case you need something to convert the TTL Uart into a USB.
Also most boards provide a number of ADC channels (even the $50 robot circuit on this site)! Not sure if you need 6 or whether thats just an example.
You then need to see what kind of cable you might need to program the board. Some boards come with a cable but for most you need to buy one, or make one yourself, if you don't have one.
Finally you need to think about programming languages. Most boards use either C or Basic. Which are you familiar with? The choice of language also dictates the software tools you need on your PC in order to write and compile the program. Some boards, but thankfully few, charge you for the development environment.
So think about your budget, and then make sure you've got the answer to all of the above before you take the plunge and buy ANYTHING. If you will end up owning the hardware then you may want to stretch your budget to something bigger than you actually need so that you can re-use it afterwards for other things.
I suggest you also look at the SoRScope project on this site. This could be used on (ATmega chips with C) to capture the ADC data and xmit to the PC where it can be shown just like an oscilloscope. This means you could see the output from your microphones before you've even written a line of code.
