Best answer by Mr.Red
As far as I know, if you ad a couple of Sonos One as back speakers with the playbar and the sub, you can control them with Alexa.
Another option could be installing 2 Play 1 and an Echo dot.
Take a look at this review of the Sonos One, the guy sends music to the PlayBar in other room using a Sonos One.
https://www.youtube.com/watch?v=cTNXmq7FvSE&t=629s
Best regards