Hi Mick, it’s possible different platforms/devices have different original sound control locations.
I use my PC windows/chrome and when I enter the zoom room at the very top left hand side of my screen there’s a little dialogue box, saying original sound off/on
Worth a look 
No paid membership required, I believe I am the only licence holder here and when i enable others ti use Original sound it should show up for you and others. All the best.
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I can’t find it either 
Using the Linux client.
Not sure how it works on linux but you don’t need premium. You click little arrow near your microphone icon while on meeting, go to Audio settings at the bottom, under Music and Professional Audio you hit Show in meeting option to enable Original Sound.
what happens when you go over to advanced settings?
just checked this Iain:
Hi @appalloosa-cob, it looks like from the 5.5.0 release, you are able to share mono or stereo sound. When sharing content and wanting to share computer sound as well, users can select between mono or stereo audio. Only the user sharing their audio will need to be utilizing this client version or higher.
However, within the Client settings under ‘Audio’, it doesn’t sound like you have the settings for ‘Music and Professional Audio’ on Linux.
Unfortunately, original sound and stereo aren’t currently supported on our Linux client at the moment. Feel free to mention your request for this feature as it definitely helps for us to see this within the near future. You can send your feedback to: Feedback - Zoom
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Addendum …
also added to post 1 above
Background & Effects
Please ensure Background is set to None for optimum clarity when performing.
On no account should you use the Blur virtual background.
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