I haven't seen any How To's about such things on the ACX site,
No and that's precisely where we had our little discussion. We live in details and specifications and they tend toward broad overviews and generalities.
I haven't found it yet, but we have actual technical tests to make sure your performance is in the ball park. I have never seen anything like that on the ACX system. I think you're supposed to submit and pray.
If you watched through some of the videos, there are sections devoted to the construction details of your sound proof booth. Like, of course, you have one. Actually, Ian on the forum (also in LA) does. That's how he got his first novel published. I have a particularly quiet third bedroom and that's how I shot my sound tests.
It's not popular to say this, but there are setups and locations that will never
produce ACX compliant sound files. Everybody on the forum shows up with the idea that we're going to rescue their performance. All the tools we use to help do that are featured in a lecture slide called "Tools to never use."
Would those infrasonic frequencies mess up .mp3s for audiobooks, btw?
My opinion is yes. MP3 encoding breaks up a sound performance into characters, sounds, overtones and harmonics. Then it deletes the quiet ones and puts the show back together. So no, it doesn't just make the show muffled. That was the trick. Few people can tell they're listening to a compressed show until you either screw up the compression or directly compare the show to the original.
If you have significant trash too low or too high, those sounds are directly competing
with the high quality actual performance for compression attention. If you have rumble, for example, and it's louder than the overtones of the violin you paid a lot of money for, you now have a lower quality violin and perfect rumble.