Web Audio API - Get correct frequency

I am using the Web Audio API to get the frequency of the sound, which is coming from the microphone. For this I found some useful code on this github repo: http://ift.tt/2mUdPkl .

Now my problem is, that I am getting a lot of different frequencies, for only one little sound. This sound might be from my voice, or from an instrument.

These frequencies are like between 90 and 4000Hz. But as I know, one note of a human voice or from an instrument, only can have one single frequency amount in Hz. And I am pretty sure, that I am only playing one single tone.

So how can I know, which frequency of the 3 or 4 frequencies per tone is the one I am searching. I need this value, to recognize musical notes like C, D, E from their frequencies. I hope this question isn't off-topic, because I really tried hard to find a solution and I don't know if this is a solveable issue from the API itself, or if I have to eliminate some frequencies somehow. I would appreciate any kind of help.