Demodulating wideband FM signal in Octave from recorded baseband IQ samples
This should work if you further filter to 15KHz after your FM discriminator. FM stereo is actually sub-modulated as follows:

L+R: 0-15KHz

Pilot Tone: 19KHz

L-R: 23KHz-53KHz as DSB-SC on a 38 KHz sub-carrier

Where L is the left channel and R is the right channel.

Update: I found a nice graphic showing this from Keysight with further details on FM broadcasting at this link: http://rfmw.em.keysight.com/wireless/helpfiles/n7611b/Content/Main/FM_Broadcasting.htm



If the pilot tone is present, that indicates the station is transmitting stereo but with this approach mono receivers would still work as you would hear the combined left and right channels. 

So the composite signal is formed by first AM modulating using Double-Sideband-Supressed-Carrier (DSB-SC) the L-R channels onto a 38 KHz sub-carrier (subtract R from L and then simply multiply this with a 38Khz tone).
The tone is frequency divided by two and this coherent 19Khz pilot tone is added to the DSB-SC L-R waveform. Finally R is added to L and this baseband signal is added.

This is the signal that is then FM modulated, so would be the signal you would expect to get if you are properly FM demodulating.

A stereo demodulator after FM demodulation locks onto the 19KHz pilot to create the AM sub carrier at 38Khz (2x) and with that easily demodulates the AM DSB-SC sub-modulation (by multiplying the two) to achieve L-R at basebnad, and then can extract L and R by summing and subtracting the L+R and L-R results. 

If you want to do a simple test of audibly hearing the demodulation of the FM signal, filter out the 19 KHz pilot and higher frequencies after you have FM demodulated this composite signal and you will hear the combined L+R channels.