[quote=“guessed, post:181, topic:169644”]The serviceId string is incorrect. ServiceId strings have :serviceId: in them, not :device:. They are sometimes different in other ways also.
“hover” your mouse over the label of a Sonos-specific variable in the Advanced Tab of the device to see what the string really should be… the bubble help for the Variable’s Label will let you know the serviceId.[/quote]
Thanks for the response and the pointer as to where I should look.
Unfortunately none of my three Sonos devices have any variables in the Advanced Tab other than Configured = 0 and Last Update = 0, and those don’t do anything when hovered over.
I did a bit of Googling and came up with what I think is the serviceid that I need: urn:micasaverde-com:serviceId:Sonos1
I modified the code (as below) and retested.
local DEVICE_NO = 27 – the Office Sonos device number
local LS_SID = “urn:micasaverde-com:serviceId:Sonos1”
local lul_arguments = {}
lul_arguments[“newSetFileToPlay”] = “//FND/Music/Doorbell/Tubular_westminster_8.mp3”
lul_resultcode, lul_resultstring, lul_job, lul_returnarguments = luup.call_action(LS_SID,“SetFileToPlay”, lul_arguments,DEVICE_NO)
local lul_arguments1 = {}
lul_resultcode, lul_resultstring, lul_job, lul_returnarguments = luup.call_action(LS_SID,“Play”, lul_arguments1,DEVICE_NO)
return true
But that doesn’t work either.
Still, a step closer perhaps… 
Thanks for your help.