It’s a bit of a coincidence that the signal is in the audible range, for sure! I’m not too surprised that an MP3 can reproduce a digitally modulated sin wave in the audible spectrum, but I can see how it’s surprising to people that a sound card is basically a dinky low frequency SDR. Van Eck Phreaking is another good example of this kind of stuff. CRTs in particular produce very obvious emissions which match what’s being displayed.