getByteFrequencyData results in a normalized array of values between 0 and 255.
(it copies the data to the array it gets passed-in).
the frequency bands are split equally, so each element N of your array corresponds to:
N * samplerate/fftSize
You are passing the result of executing the function instead of the function itself. Since the result of the function is undefined, you are executing checkIfGameAlreadyStarted and then passing undefined to setInterval which doesn't do anything.
Instead of this:
Your statement should be this:
without the parentheses at the end of the function name.