It would have been nice to show the FM modulation equation in use along with the code. I say this because most people underestimate the complexity of FM modulation and instead write an approximation that doesn't create the hoped-for result.

Here's a reference for the actual math:

http://arachnoid.com/phase_locked_loop/index.html#Detailed_D...

(search below the provided bookmark for "Frequency-modulated reference oscillator")