With the Arc and Era 300s setup as surrounds you do not have an available input for any audio device. When the 300s are paired as surrounds, Bluetooth, line in and the front speakers are disabled. 

You can purchase a Port to connect to an output on the mixer, but you will have a delay between the performers system and the Sonos system. This is because of the signal processing that needs to be done within a Sonos device before you hear the audio.

If the Sonos is in a different room, you may not notice the delay, but if the performer can hear the sonos speakers, it may be a little distracting.

Thank you for your response, what if i disable the surround? Will that give me another option? 

Then you can use the inputs of the Era-300s BUT the delay is going to drive a live performer nuts. A lot of folks find it hard to even talk with the delay, much less sing and play.

Rent a nice PA or Karaoke system for a few bucks.

The delay will be a killer if performers can hear your Sonos speakers. If you are determined to go this route, you will need the Sonos line-in/USB-C adapter if you’re not able to use Bluetooth- which is another possibility for introducing additional delay!