If you leave the audio at 100% and you deactivate it the problem persists. This problem disappears only if you activate the Mi Sound Enhancer. While using the headphones, or in general the headphones jack (might that be via Aux cable to a speaker, a car, earphones, headphones, etc), and setting the volume up to 100% at maximum, the audio starts to crackle. MIUI Version: MIUI Global 11.07 | Stable 11.0.7.0(PFDMIXM)
I think I have discovered a small (not really) bug.