Matlab FFT keeps plotting mirrored values
It is a mathematical property of the DFT that its output is conjugate symmetric if its input is real-valued. That means that for the magnitude of the DFT output signal $X_k$ of length $N$ it holds
$$
|X_{N-k}| = |X_k|.
$$
So what you see is to be expected. (In your example $3200\,\mathrm{Hz}/2 - 266\,\mathrm{Hz}\approx1330\,\mathrm{Hz})$ If you're only interested in the positive frequencies, just plot the first $N/2$ elements of the DFT output, the rest is redundant.