Try ‘Alexa, stop on (speaker room name)’ 

I often don’t need to use the room name, depending on whether I’ve used Alexa for something else. 

‘Pause’ is also likely a better word than the phrase ‘switch off’.

‘Switch off’ does not work because Sonos speakers don’t switch off.

I normally use "Pause" or "Stop" (seem to do the same thing!).

I mainly use ‘Stop’ and ‘Pause’ aswell,

I guess you could feedback your preferred instruction to Amazon to use ‘switch off’ for any playing music and see what they think, or perhaps even easier, you could create an Alexa App routine to stop all smarthome speakers when saying "Alexa, Switch off”, as per the ‘mock’ screenshot attached.

I love the idea of creating a routine that enables you to say "switch off" in order to stop the music, rather than say "stop".

Apparently the wife of Tim Berners-Lee, the guy who invented the Internet, told him that "www" would never catch on, as it took longer to say than "world wide web". So maybe that routine has a future.

ha ha I was just trying to cater for those that may not like change…ha, but it’s a fair comment.

My supervisor constantly refers to “Middleware” as “MW” (Em Double-U), which is one extra syllable.  It drives me bonkers.

Ken - I.knew you could be trusted to take my comments in good spirit.  You didn't let.me down ;)

“Switch off” used to work, and is still working in other skills, just not when using TuneIn. Tried “pause” and “stop” with no success.

Going to try the routine next.

In that case there is something wrong. Have you tried disabling and re-enabling the Sonos skill?

Routine didn’t work, but here’s what I’ve found.

“Stop”, “pause”, “switch off” works in all skills except TuneIn.

“Stop ncurrent room]”, ie. “stop bedroom” is the only command that works in TuneIn.

Thank you for all the suggestions, much appreciated!