Is it possible to Frequency modulate the internal VCO of the HMC829 PLL?

Yes, a two point modulation scheme can be used by installing a very narrow loop filter, sub-100Hz with very low CP current.  The audio is injected (AC coupled of course) into the VCO at the Vtune pin.  The audio path would also have some series resistance that, when combined with the loop filter resistor acts as a voltage divider on the audio signal.  The Kvco of the HMC829 VCO is nominally 12MHz/V (but can vary, worst case, from 8 to 20MHz/V), so for a wideband 75kHz deviation system, the voltage deviation at Vtune only needs to be 0.075/12 V = ~6mV.  The resistive divider would drop the much higher amplitude audio signal to this required range.

Trying to modulate the HMC829 for audio by successive updates to the frequency programming registers (to generate the instantaneous frequency deviation) is not recommended because the perpetual SPI activity will cause spurs and noise to appear on the VCO.

Yes, a two point modulation scheme can be used by installing a very narrow loop filter, sub-100Hz with very low CP current.  The audio is injected (AC coupled of course) into the VCO at the Vtune pin.  The audio path would also have some series resistance that, when combined with the loop filter resistor acts as a voltage divider on the audio signal.  The Kvco of the HMC829 VCO is nominally 12MHz/V (but can vary, worst case, from 8 to 20MHz/V), so for a wideband 75kHz deviation system, the voltage deviation at Vtune only needs to be 0.075/12 V = ~6mV.  The resistive divider would drop the much higher amplitude audio signal to this required range.

Trying to modulate the HMC829 for audio by successive updates to the frequency programming registers (to generate the instantaneous frequency deviation) is not recommended because the perpetual SPI activity will cause spurs and noise to appear on the VCO.