Sonos One - Adding Alexa

Sorry for the long post but this type of issue is normally down to one of three things. In your specific case, I suspect section two:

1/ Duplicate device names - Alexa prefers that names for Devices, Groups and Scenes are unique,
1.1/ Please open up the Alexa App>Smart Home and make a note of any duplicate device names;
1.2/ You can then change them in the source app. e.g. Use the Sonos App to change the name of Sonos players. (I personally use the naming standard Room Name + Device type. e.g. Kitchen Sonos, Kitchen Echo Dot, Lounge Sonos, etc)
1.3/ Once you have changed any duplicate names then please In the Sonos app, select Settings>Room Settings and for each of the Sonos One's sign into 'Voice Services' and select 'Remove Amazon Alexa from this Room';
1.4/ In the Alexa app, select the Alexa Icon in the centre of the row of Icons at the bottom of the screen and ask ‘Alexa, Discover Devices’.
1.5/ Here is a link to my naming standards: https://en.community.sonos.com/amazon-alexa-and-sonos-229102/best-practices-for-naming-of-devices-6819283/index1.html

2/ Some form of Account Linking or Authorisation has not completed successfully. There are a number of causes for this and it's best to try these steps 1 at a time and test before moving on to the next step if the problem persists:
2.1/ Open the Alexa App>Skills>Your Skills>Sonos and 'Disable' followed by 'Enable';
2.2/ Ensure that you don't have multiple Amazon accounts logged into different Apps on your device, as this can confuse the Amazon Authorisation. If yes, then please log out of the other accounts and then complete step 2.1 again;
2.3/ If the problem still persists, please logout of all Amazon accounts then select the Alexa App>Skills>Your Skills>Sonos and 'Disable. Now power down all Sonos devices for 10 secs, allowing for any wired devices to boot before booting the remaining Sonos players. Now log back into the Alexa App>Skills>Sonos and 'Enable'

3/ If the problem persists then an entire reset of the Authorisation & Account Linking process is required:
3.1/ In the Alexa app, select Skills>Your Skills>Sonos and then 'Disable';
3.2 In the Alexa app, select ‘Devices>All Devices'. Now select each Sonos Device in turn and click the Trash Can in the top right of the screen;
3.3/ In the Sonos app, select Settings>Room Settings and for each of the Sonos One's sign into 'Voice Services' and select 'Remove Amazon Alexa from this Room';
3.4/ If you have any wired Sonos devices, please power down for 10 secs and then switch on and wait for it to boot;
3.5/ Now power down all other Sonos Players for 10 secs then switch them back on and wait for them to boot;
3.6/ In the Sonos app, Select Manage>Add a player or sub' and add the Sonos One's again. This will complete the set-up process again;
3.7/ In the Alexa app, select Skills>Your Skills and ensure the Sonos skill is 'Enabled', if not please 'Enable';
3.8/ In the Alexa app, select the Alexa Icon in the centre of the row of Icons at the bottom of the screen and ask ‘Alexa, Discover Devices’. Ensure that your Sonos One's are displayed correctly and not shown as 'Off Line'.
3.9/ Also check one last time that there are no devices with the same name. Remember, if there is a need to rename any then you'll need to carry out step 3.8 again.

Let us know how you get on.

Thanks for such a detailed reply.

I started to follow your instructions. Checked the names are not duplicated in the Alexa app. Then went to the Sonos app Settings > Room Settings. When I clicked on Voice Services here it let me sign in and setup Alexa in each of the speakers. It is now all working fine.

Not sure if it didn't like trying to set them both up at the same time before, or if it was just a network error that fixed itself. But thanks for the help!

Lee_vincent I hope it works for you too.