Arduino clone, and unwanted noise
I used Arduino clones few times without issues, it's open standard with no extra fancy parts, so unless it's really poor one and/or you're very unlucky all tends to be fine.

As for floating ADC reads this is rather typical behaviour, this is effect of noise on power supply voltage. This applies especially when microcontroller is powered from USB port of your computer or wall plugged power supply, but even on batteries track on your PCB can pick up noise (for example from nearby electrical devices. If your application needs more precision (for example alarm triggered by noise detector) you can take a few measurements, calculate mean value and use it instead.

I don't really understand "1 to 3 integers" part of your problem. If you mean that plugged microphone gives one digit reads (1-9) and when it is knocked with finger signal is two orders of magnitude stronger (3 digit number, 100-999), then this is actually what I would expect.