Your January sample does not have any voice frequencies above 8kHz …
Librivox specifications are 44.1kHz sample-rate, & (constant) bit-rate of 128kbps (mono) https://wiki.librivox.org/index.php/Tech_Specs
That combination permits frequencies of up to 20kHz.
Possibly you have used a 16kHz sample-rate at some point during the recording/processing,
that would remove everything above 8kHz.
The other librivox recordings I’ve (randomly) looked at use at least 10kHz, (apparently distributed as 22050Hz 64kbps mp3).
Trebor, I am now officially out of my depth . I know what “Hz” is, but have no concept of what it means with respect to my recording of my voice.
I am still at the point of trying to make my audio recordings within a specified decibel level!
I will have to come back to this Hz business at a later date.
Possibly you have used a 16kHz sample-rate at some point during the recording/processing,
that would remove everything above 8kHz.
My January 2nd recording for LibriVox was to have them check that my hardware/software setup was a good profile.
Apparently it was.
Since then I have thought of two suggestions (for LibriVox)
(1) Besides the hardware/software settings, institute a second one-minute test for things like volume (amplification), pauses (between words, sentences, paragraphs etc), that is, those personal traits that are not picked up by examining a wave, but can be adjusted with experience.
LibriVox are very receptive and do not demand a flawless accent; with my mongrel tones from English prep school, Aussie goldfields, and Canada/USA, I am (gratefully) proof of that tolerance.
(2) Someone who records for two different audiobook publishers would have two different hardware/software setups and flip back and forth. Periodic hardware/software checks might be a good thing.
LibriVox does have a Checker.EXE program that inspects loudness, bit rate, all lower-case letters in filenames and so on. The author of Checker.exe has left the forum, so Checker.exe is an unsupported program right now.
This all leans me toward an attitude (on LibriVox) of “If it ain’t broke, don’t fix it”.
The sample-rate defines the maximum frequency that can be recorded, and is specified in Hz.
Your January example mp3 has a sample-rate is 44.1kHz, with bit-rate of 128kbps,
so is exactly as specified by librivox, however you’ve only used half of the audible frequency-range available: there’s nothing above 8kHz.
Somewhere in your recording/processing chain something has removed frequencies above 8kHz.
e.g. if your recording-device was set at a sample-rate of 16kHz, that would filter-out everything above 8kHz.
To avoid that in future recordings it’s just a matter of resetting the recording-device sample-rate to 44.1kHz.
If you are recording using a computer, Skype/Zoom type applications could be responsible for the 16kHz sample-rate.
. I know what “Hz” is, but have no concept of what it means with respect to my recording of my voice.