In GNU Radio, FM demodulator is implemented digitally as
a = present_complex_sample * conj(previous_complex_samples)
out[current] = some gain * arctan ( imag(a), real(a) ); //demodulated FM
So, you can also implement the same for demodulating FM signal in
Recorded FM files may already be available in internet. So, use that
and test your MATLAB code.
For connecting USRP to Matlab, toolbox is already released by Mathworks
from 2011 versions. If using older version of Matlab, you can use
http://www.flexible-radio.com/tools4sdr/download third-party plugins.
But, you can only use RFX 900 board which will not support FM reception
Hope this might help you,