It's probably due to the unusual sound format, 24kHz mono PCM, and the fact that it was somehow forced into a WebM container, which only supports Vorbis and Opus officially.

It looks like the author created it using the "higher quality" ffmpeg command line, except for the "webm" final extension, producing the opposite of what's described as "an MP4 file that's compatible with more devices".

https://github.com/denizsafak/abogen/tree/main/demo#for-high...