https://www.sonos.com/en-us/shop/port?utm_campaign=rta_sonos_pmax_us_feed-only&utm_medium=cpc&utm_source=google&utm_content=rta_sonos_pmax_us_feed-only&utm_term=&gclsrc=aw.ds&gad_source=1&gbraid=0AAAAADo4HCeS28bSpBQ7GJ0qlblnXucBY&gclid=CjwKCAiAg8S7BhATEiwAO2-R6h_ppdml7i8uDQizYeUP1aFQL6cGOVfMCebn6IA0t6I0zySogWCQ2BoC5uQQAvD_BwE

Frankly, Sonos won’t drive a third party set of speakers in a 5.1 setup. If you want to use third party speakers, the best you can use is 4.1, with the interpolated center channel. 

You can stream stereo music into your 5.1 setup using a Port.

I’m guessing Amp if speakers done have an amp?

You cannot connect a single Sonos Amp to a 5.1 speaker system, the Amp can handle two front speakers and compute a “phantom” center. It can also command a non-Sonos subwoofer. For rear speakers you need. Another Amp.

If you don’t have an amplifier for your existing built in speakers then you’d need one.

As said, if you are happy with Stereo and only driving four speakers the Sonos Amp would work. You could also drive an active sub, not a passive one, from the Sonos Amp.

If you want all five playing and 5.1 sound you need a 5.1 decoder/amp to match your speakers. Maybe just get a 5.1 AVR and a Sonos Port to feed it.