Diagnosis
This is almost always caused by interference on the audio signal. It can be from other wireless devices (like routers, cordless phones, microwaves) or a low battery on the parent unit causing poor signal processing.
How to Fix It
Start with the channel change, just like for video issues. On the parent unit, go to Menu > System Settings > Video Settings > Change Channel. Cycle through the options (1, 6, 11 are best) and listen for the cleanest audio. Also, plug the parent unit into its charger. Audio distortion is often the first sign of a weak battery struggling to power the radio.
If That Didn't Work
The issue might be with the camera's microphone. Check for any visible debris or dust covering the small microphone pinhole on the camera. Gently use a can of compressed air (held upright) to blow across it—don't insert the nozzle. Also, in the parent unit menu under 'Audio Settings', make sure 'Sensitivity' is set to High or Medium, not Low.
Heads Up
If the crackling is rhythmic or constant, and changing channels does nothing, it could be a hardware fault in the parent unit's speaker or amplifier. Before assuming the worst, test by plugging headphones into the parent unit's jack. If the audio is clear in headphones, the internal speaker is likely failing.