Just sharing in case this helps others…
As a newbie, I am trying to make sure everything is done as well as possible, so after benefitting from a 10 second test here (thank you!), I found that I could also send a longer test to ACX for feedback.
Here are the instructions I received from ACX:
“I understand you would like to submit a sample audio file to our Audio Team for review. This survey only accepts mp3s no longer than 10 minutes, at a 320kbps maximum bit rate. To begin this process, please take the survey located at the following link: To begin this process, please take the survey located at the following link:
https://audible.az1.qualtrics.com/jfe/form/SV_265YEkshVJXHqPX?c=
Taking this survey will help the Audio Team provide the best feedback for your sample. The survey will first guide you through a few short questions regarding your sample, current production setup, and skillset. At the end, you will upload your audio sample and provide contact info so the Audio Team can reach out to you with their feedback. Once you have completed the survey, the Audio Team will review your sample and provide feedback within 5-10 business days.”
I filled out the survey, sent in the sample, and got back a very nice reply several days later. I was hoping that they would provide feedback on a raw sample, and that’s how I tried to set it up in the survey, but they must not be set up for that. Their response was to tell me that I needed to put the requisite periods of room tone at beginning and end, and several specifics that boiled down to “master the file”. So, I’ll soon be sending them another sample that meets those requirements, and hoping for a good report. 
J
