Actually, that is not really true. There are really no measurements that would apply to things like "break-in" on audio equipment that we have now that we didn't have at least 50 years ago.

Things like jitter measurments used on digital audio equipment have existed for decades, but have just been applied to other areas of electronics. The jitter measuement wasn't invented just for measurement of CD systems.

The mechanisms of change in the characterisitcs of circuitry over time have been known almost since the beginning, and solutions found for cancelling them - if they were'nt, reliable electronics could not possibly exist as they do today. Just think how unreliable things like aircraft would be if the electronics constantly underwent "break-in", changing their characteristics at the whim of the magical silicon's mood.

Another fact that that is not mentioned and often misunderstood about modern solid state electronics is that it uses, almost without exception, massive (and I do mean massive) amounts of both local and global negative feedback. One effect of negative feedback is to cancel any changes in the circuit's characteristics. The result is a stable and rock steady circuit, which does not change over time (i.e. "break in").

This is not true of some vacuum tube circuits such as my single ended triode amplifiers, which do not make use of global negative feedback, but only small doses of local negative feedback. In circuits like these, the characterisitcs and sound of the circuit are at the whim of the aging of the emmissive coatings on the cathode of the vacuum tubes. However even given this, any actual change in the sound of these amps is so gradual and subtle that I cartainly cannot hear it. Obviously, this does not apply to solid state electronics at all.

No, certaian fields are vastly more understood than other areas of science. You can't compare the understanding of one science to another directly.

Audio electronics is vastly simpler than other areas. Things liks analog to digital conversion existed long before digital audio did. There are'nt any magical mechanisms going on just because the electrons represent music.