You cannot play two sources at once.  It's either Bluetooth or Line-In, not both.

Hello jgatie - understand that - however to be clear the set up is as follows - karaoke mixer is connected to karaoke mike via UH frequencies and to MacBook via bluetooth , then a line-in cable is connected from mixer out to Sonos era via a converter cable ( 3.5mm to usb c) - should this set up not work? I called Sonos and they just asked me to buy their 20 dollar converter cable vs the 3rd party I bought of amazon as the line-in cable is not shown ‘live’ in Sonos app. I wanted to be sure that it's my little 3.5 to usb c converter that is a problem and not anything else. Unfortunately I don't have any other speaker ( I have apple HomePod but they are closed systems to apple ) … thanks 

You cannot use a generic 3.5 mm to USB.  Those generic adapters are DAC (Digital to Analog Converter).  You need something to do ADC (Analog to Digital Converter) and the only one anyone has seen is the Sonos adapter.  Note also, the Line-In is going to cause a 75 ms delay in order to buffer the input for multi-room distribution, which makes it pretty useless for live performance such as DJ’ing or Karaoke.

It's a single Sonos speaker in single room, not multiple - so the 75ms delay should not be a problem - unless I have not understood you … 

The 75 ms is there no matter how many rooms are playing.  You cannot bypass it.  Sonos just isn’t fit for purpose in regards to live performance. 

well, well, then I probably pick up 20 dollar old used PA speakers of ebay , damn Sonos  !!