The switching frequency is well above the limit for audio frequencies, and a good quality PSU will do plenty of filtering. USB to S/PDIF is good for a few reasons: 1) The signal is kept digital, and either differential, balanced or optical for as long as possible. This makes it hard to pick up noise. Digital-to-analog conversion should be done as late as possible because digital signals are very noise-tolerant. 2) Optical connections eliminate grounding issues (less of a problem if you obey #1, anyway). 3) S/PDIF equipment tends to be built to higher quality because it's considered somewhat "high-end" (or, at least, more than a cheap motherboard audio chip). 4) D-A conversion is done far away from the noisy CPU and data buses. It has nothing to do with the SMPS.

You should hear the sound from the vacuum tube guitar amplifier I built. The high tension (300V) is generated by a crappy 50kHz switching boost converter I designed and built myself. The noise on the supply is absolutely awful - 1Vp-p even with only a light load. You can't hear it, though, because it's 50kHz. And that's really low frequency for a SMPS.

Not responsible parents.

OK, not a bad idea. Couple problems, but it could work.

And here is where it falls apart. People will start ignoring the signal again as soon as those who don't need to use it start to use it anyway.