So i just tried this, and i can't find anywhere in the documentation as to why, but when using an audio tag, the url can not have a hyphen in the file name.
using the same file, just different names, and these are the results.
Does not work
<audio src="https://cdn.contoso.com/bot/audio/contoso-intro.mp3" />
<audio src="https://cdn.contoso.com/bot/audio/contoso_intro.mp3" />