How to determine the frequency of each row in a MATLAB spectrogram
The physical frequency $f$ that a given Discrete Fourier Transform (DFT) frequency bin $k$ corresponds to is $f = \frac{k}{N_{DFT}} \cdot Fs$. 

For an $Fs=44100$ Hz, the middle point of a given $N_{DFT}$ point DFT would correspond to $\frac{1}{2} \cdot Fs$, also known as the Nyquist Frequency.

EXCEPT MATLAB, which maps $1$ to $\frac{Fs}{2}$.

For more information, please see here.

Hope this helps.