Hi Richard,
I suffered through understanding the quadrature mod/demod block settings a while back. I provide some links to that information below. I provide a formula for the settings for both digital and audio applications. As you can see (from the reading), the settings can be derived from samples-per-symbol and modulation index or deviation and baud rate. Or just peak deviation and sample rate for audio.
The quadrature demod gain is really just a gain, btw. If you set it right, your demodulated audio will have unity gain, which is nice/necessary for most of the clock synchronizers.