From what I have seen, Amazon isn’t supporting Alexa + on ‘Alexa Enabled Devices’ (which is what Amazon calls 3rd party devices with Alexa mics) currently.  However, you can use Alexa + to control Sonos speakers.  In other words you won’t be able to use Alexa + when talking to a Sonos speaker, but you can tell an echo to play music on on your Sonos speakers.

Neither Amazon or Sonos has said anything about future support.  I think that’s going to depend a lot on how well Alexa + is received by consumers, which seems to be a bit of an unknown at this point.  It then comes down to whether it’s technically possible, and what sort of licensing costs are involved.  I would guess that Amazon wants this new feature in as many homes as possible, so will keep entry costs low, but the $20/month or Amazon Prime subscription fee doesn’t seem to imply Amazon feels the same.