Your test sounds fine to me, but it fails the Checker test.
The problem is that you have 48,000 Hz instead of 44,100 Hz in the Mp3 format.
You fix it by clicking Edit in Audacity, then choosing Preferences, then Quality, and you'll get a list of numbers from which you choose 44100.
You might like to submit the test in Listeners Wanted to get input from people with more technical expertise than mine. Larry might have some suggestions, too, but the file sounds good to me. It is within the desired volume range of 86 to 91 dB: I don't hear any distracting back ground noise, and you read well. When you get it to pass Checker, just ask again here for the section you would like to read.
You might like to access the Checker tool yourself by clicking on the three white lines in the top left corner of the forum index and choosing Guidelines for Listeners and Volunteers (Librivox wiki), then going down to Recording under the larger heading Volunteering. The Checker is the first item listed under Tools in the right hand column below Recording. You download it to your computer, then drag and drop your Mp3 files into it, and it analyses them according to Librivox specs and gives you a report as to whether the file passed or failed. In a section called Validation, it tells you why a file failed or passed with warnings.
Thanks for your interest. Looking forward to you getting started recording.