Welcome n
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 MP3, 44.1kHz sample rate, mono, and between 86db and 92db, preferably 89-90. Your test audio is
192kbps bit rate. Everything else is good.
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.