If you purchase another Play:1, you will be able to add both Play:1s as proper surround speakers to the Playbase. Currently, by grouping your speakers with the Playbase, you are hearing the exact same audio from the grouped speakers as you hear from the Playbase. When the Play:1s are bonded to the Playbase as surround speakers, they will play the actual left and right rear surround channels instead. So if you are watching a movie and there is an explosion behind the camera, you will hear the explosion out of the Play:1s only rather than all of the speakers. This will give you a more immersive experience especially when watching Dolby Digital 5.1 content.
When a speaker is bonded to a Sonos home theater device as surrounds, this disables the line-in port on that speaker. So if you were to purchase another Play:5 and use both of them as surrounds, you would not be able to use the line-in ports on either Play:5. NOTE: The Play:1 doesn’t have a line-in port.
I recommend getting another Play:1 rather than another Play:5. Play:5s as surrounds is usually overkill for most living rooms. You don’t want the surrounds overpowering the Playbase.
Hi
Just a bit more info…There has never been a Play 1 Gen 2_just the Play 1 which was discontinued in favor of the Sonos One Gen1. The current speaker that is similar in design to the discontinued Play 1 is the Sonos One Gen 2.
Unless you can find a used Play 1 (which there are some on eBay) you’ll have to purchase two Sonos One Gen 2 models. The Sonos One Gen 2 is not compatible with the Play 1 as a surround nor stereo pair. You can probably find two used Sonos One’s Gen 1 on eBay as well.
The Sonos One Gen 1 and Sonos One Gen 2 can be mixed together to serve as surrounds or a stereo pair. Another benefit of using a Gen 1 and/or Gen 2 versions of the Sonos One is that they offer Alexa or Google voice control which is active even when bonded to the Play Base or Play Bar.
If voice control is not desired you can opt for Sonos One SL’s as surrounds. They may come in at a price point close to a used pair Sonos One’s Gen 1.
As @GuitarSuperstar said using Play 5’s as surrounds is overkill in most situations.
Thank you so much for your clear answer 
Your existing Play 1/5 I don’t see being easily integrated into this system so I would just use them in other rooms, or sell them. Just wait until Sonos One SL are on sale and buy 2 as surrounds, I got a whole bunch of them that I use all over the house (including as surrounds in the theater) for around $130 each. This is already a decent theater system when combined with the Playbase, but it’s closer to 5.0 than 5.1 because the Playbase subwoofer is restricted by the size and design (although quite good considering those). When you can afford it and find a sale, then drop the hammer for a Sub which will really fill it out.
