This is a recording of someone trying to explain “how to”.
… there is a distortion whistle
…The recording sounds muffled
Have you considered written instructions?
You may be able to make some improvement, but you can’t turn a bad recording into a good recording.
There’s no substitute for starting-out with a good recording. There is a good reason professional recordings are still made in soundproof studios with good equipment.
Maybe you can re-record with a better setup… You can get very good results with a “studio style” or “podcast style” USB microphone (as long as you have good acoustics in your “studio”, good microphone placement, and a good “performance”.)
Regular “computer gaming” microphones, “communictions” microphones, and the mics built into laptops are not that good.
distortion whistle when she speaks. …how can I reduce this whistle?
You can try a notch filter. But, you’ll have to find the right frequency. You may be able to find the frequency with Analyze → Plot Spectrum, but there are many frequencies at once with a voice (or any natural sound) so if that whistle doesn’t “pop out” in the spectral display, it may take some trial-and-error. It may be more than one frequency, or the frequency may not be constant.
You can also do some experimenting with the Equalizer, turning-down one frequency-band at a time to see which one affects the whistle the most. That may actually be enough, and you may not need a notch filter.
You can’t remove distortion.
The recording sounds muffled - how can I make it clearer?
Try experimenting with the Equalizer. Boosting the mid-high to high frequencies (the sliders on the right hand side) will tend to boost “T” and “S” sounds. But, that will also boost noise and any remaining whistle.