I’m completely lost by your question.
I use the playsound & say commands in rules a lot and I haven’t installed an extension anywhere.
(I route the sound events to webaudio or specific audio sinks like my Chromecast devices)
The ONLY thing that needs to be setup / running for webaudio to work is a web browser page on a (one or many) client machines, that is looking at your openHAB2 server, via…
BasicUI
ClassicUI
PaperUI
HabPanel
(I only use PaperUI and HabPanel (Inc HabPanelViewer app))
When you say “the sound continues to play on the server”, do you mean you can hear the sound? Or that you see it being triggered in a log?
If you can hear it, you must have a web browser running with an openHAB2 UI open?
I hope this helps you.
Regards,
Stuart
update
Don’t forget that the openHAB2 app or browser must be running (not sleeping) on your client machine.
My windows laptop will play audio if the browser is simply running, whereas my Android devices will put the browser to sleep if it’s not in the foreground.
I think the same logic applies to the openHAB2 app.