Getting Frequencies Corresponding to Peaks In FFT Plot - MATLAB
MATLAB's indexing begins at 1, instead of 0, as most programming languages use.  This causes a little wrinkle in interpreting the fft results.  Index 1 corresponds to bin 0, index 2 to bin 1, etc.  So to get the bin number, subtract 1 from the index.

The bin number corresponds to the frequency in units of cycles per frame, where frame is the section of the signal your are analysing.  Usually, the variable $N$ is used to denote the number of samples in the frame.  You also need to know the sampling rate, usually denoted by $f_s$ which usually has units of samples per second.  CD quality audio has a sampling frequency of 44,100 samples per second.

To calculate the frequency in hertz (Hz) of a bin number, usually denoted with a $k$, do the following calculation:

$$ f_{HZ} = k \cdot \frac{f_s}{N} $$

The units for the equation goes like this:

$$ \frac{cycles}{second} =  \frac{cycles}{frame}   \cdot  \frac{ \frac{samples}{second} }{ \frac{samples}{frame} } $$

Hope this helps,

Ced