Its because steam blocks it in their CSP, sadly lots of CDNs do.
I could individually add each one to the FQDNS list, but like you said that's a lot of hassle and it has to be reaproved by mozzila everytime etc.
The way audio source is pulled is on page load. If it's doing it too late lots of sites won't interact with the plugin. Where as its an issue on sites with strict CSP as then it just blocks audio.
So sadly there's nothing I can do about it, without further reducing compatible sites.
Could maybe make a whitelist mode instead of a blacklist mode in the future.

I hope you revise your review as it "works good"