Designing all smps applications for 20yrs in the company I worked in I can say the the "magic" inside are the parts we do not understand.
Which is a general rule in human understanding btw.
There is a huge amount of knowledge to be learned - and most of it is irrelevant for your application supplying some audio amp. So, instead of diving into smps design it may be more expedient to learn the mechanism of noise distribution and noise filtering. The difference between common mode (asymmetrical-) noise and differential mode (symmetrical) noise. And which noise is relevant for audio - there may be some big high frequency spike noise at your smps that do not affect your audio at all because these frequencies are far above the audio band. Last but not least how immun is your amp against power supply noise?
All in all it is quite difficult for the beginner to trace the source of any noise but nonetheless is the first step to go imho.
Which is a general rule in human understanding btw.
There is a huge amount of knowledge to be learned - and most of it is irrelevant for your application supplying some audio amp. So, instead of diving into smps design it may be more expedient to learn the mechanism of noise distribution and noise filtering. The difference between common mode (asymmetrical-) noise and differential mode (symmetrical) noise. And which noise is relevant for audio - there may be some big high frequency spike noise at your smps that do not affect your audio at all because these frequencies are far above the audio band. Last but not least how immun is your amp against power supply noise?
All in all it is quite difficult for the beginner to trace the source of any noise but nonetheless is the first step to go imho.