For traditional TV and Amp/Receiver, mixing with microphone is pretty straight forward. Just connect everything to the Receiver or Amp, then it will do the mix and play out the sound to the speaker system. But for Sonos system, sound is native digital and flows thru the interlink, leaving no where to mix the microphone sound and output to the speakers. After some googling around a bit, it’s mostly said to be impossible to play TV sound together with microphone input (aka. Karaoke) with the Sonos sound system.
Well. It actually can be done with the help of some kind of ADC (Analog-Digital-Converter). In another word, everything should be analog at first, mixed together, then converted to digital, then go to the HDMI input of Sonos Beam, and finally played with all paired speakers.
Nowadays, TVs are all shipped with HDMI outputs, without RCA sockets anymore. But luckily headphone jacket still remains, which means TV sound could output analog signal thru the standard 3.5mm jack. Then a usb audio interface is required to do the mixing work, which will output analog signal (even internally digital in the mixer). Audio Interface with 4 inputs would be better, accepting two microphones and two line-in. A conversion line of 3.5mm jack to 2 6.35mm line-in jack will be needed to route the audio into the usb audio interface, which will mix the mic sound with the previous TV line-in, and output thru 2 monitor RCA jacks. The RCA lines will convey the mixed analog sound output to the ADC.
Some cheap standalone ADC might not work correctly to generate the encoded audio to Beam, which only accepts stereo PCM and Dolby Surround format (DTS now also supported). Another USB audio interface Behringer UCA202 can do the ADC job. Connecting with a computer (or Raspberry Pi), it will digitalize the audio signal again and output to TosLink optical interface perfectly. With the Sonos Beam TosLink to HDMI adapter, Beam will finally and successfully play the mixed sound to all paired Sonos speakers.
Enjoy the Karaoke.