One bright spot. I expect your new microphone and mixer to work. We just have to figure out the settings.
Read through the chapter and be sure you put that hold your breath and don’t move segment at the beginning. That’s called Room Tone. ACX needs Room Tone as part of the submission requirement and we need it for the filters and effects.
When you get to the end of live reading and before you do anything else, File > Export > Export as WAV. That’s the safety backup. There are no options for this format. This is the first time you need to settle your bookkeeping.
Everybody knows what this is.

It’s for sound synchronization, etc, etc, but less well known is the bookkeeping. Somebody looks at you weeks later and says: “I need everything we shot for the Scene “I Do Stuff.” Where is it?”
It’s your job to know.
Filename hygiene tells us it’s OK to use Upper Case letters, lower case letters, numbers, underscore and -dash- in filenames. No Other Characters. Today is 2020-04-18. No slash marks.
Back to Audacity. File > Save Project > Save Project. Use the filename rules and call it something you’ll remember.
Apply Audiobook Mastering.

If you’re missing some of the tools, you can get them from the instructions.
https://wiki.audacityteam.org/wiki/Audiobook_Mastering
You’re getting power line hum and buzz from somewhere, so I applied US Hum Remover.
de-hum.ny (2.11 KB)

Followed by medium Noise Reduction.
If you’ve never used Effect > Noise Reduction before, it’s in two steps. Drag-select a portion of your Hold Your Breath and Don’t Move (Room Tone).

Effect > Noise Reduction > Profile.
Then, select the whole piece with the SELECT button on the left and actually apply Noise Reduction according to these settings.

You should be able to inspect the piece with Analyze > ACX Check and it should pass like this.

Piece of cake, right?
Then you can get to work editing out your mistakes and fluffs.
You can zoom in and out to see accurate edit points and beginning and endings of words. There are thousands of Zoom tools. I only use three.
Drag-select some of the blue waves and zoom into them, Control+E. Zoom back out to the full show, Control+F. Zoom out a little bit Control+3.
When you get everything in perfect order, File > Export > Export as WAV a file called Edit Master followed by the names you settled on.
Then File > Export > Export as MP3: 192 - Constant Bitrate.
That’s the one you actually send to ACX. They’re not interested in the WAV files and you can’t ever edit or change the MP3 once you make it.

ACX has a way for you to send them a test rather than taking a chance on the whole book failing in one go.
They changed it since the last time I looked.
https://audible.az1.qualtrics.com/jfe/form/SV_265YEkshVJXHqPX?c=
Koz