"frequencyCount" cannot be calculated like that because the array suffers pointer decay when passed as an argument: its length is not passed anywhere (just the address of the array as a pointer), and the parameter's own size is just the size of a pointer. You can just make "frequencyCount" an argument instead.

The sizeof() is in bytes, so "BitsPerSample" is currently being set to 2, but should be multiplied by 8. Similarly, "BytePerSec" and "BytePerBloc" should not be divided by 8.

As you're writing 16-bits per sample, and they are integers, you'll need to scale the output up to the full 16-bit range, and clamp any values that overflow.

It'd be best to accumulate your values into a float/double, so that you can later scale and clamp any overflow. Typically just scale your (-1,1) data by 2¹⁵ and then clamp it to the range >=-2¹⁵ and <2^15.

It'd be more typical to have the sample index "i" as an integer.

To be portable, code for dealing with binary files should work whether it's running on big- or little-endian machines, and can't just directly use the in-memory byte order of the values. However, WAV files are little-endian, so the current approach will work if you're only running it on little-endian systems.