Greetings Christina
I reviewed your test sample, and while it sounds good there are some technical changes needed. LibriVox requires audio that is 128kbps constant bit rate, 44.1kHz sample rate, mono, and between 86db and 92db, preferably 89-90. Your test audio is 141kbps variable bit rate, stereo, and volume just a bit low at 85.7db.
Make sure the the input gain slider in Audacity (upper right with the little mic icon) is well to the right. Then adjust the input gain on your mic so your raw recording has wave forms that peak at about the 0.5 marks in the Audacity window:
If necessary, adjust the volume level before exporting by going to Effects>Amplify.
When exporting your MP3 from Audacity, be sure to check
Bit Rate Mode - Constant, then select
128kb from the pull-down menu, and check
Force Export to Mono.
You can pre-check your audio yourself by downloading the free Checker app:
https://cgjennings.ca/projects/checker/
Passed with Warnings is a fail with regard to volume.
Please make a new test recording and post it in a reply to this message. I will come back and review it for you.
