Sonos speakers are designed to be used with other Sonos speakers. But if your old hifi has an audio input, you could add a Sonos Port to it and play Sonos through your old speakers along with the Era 100.

Thank you, @GuitarSuperstar That is a super expensive solution- is it the only way? There aren’t any Line In connections from the C slot to the RCA slots on my existing amp which would work?

You can send external music to the 100 but you can’t send what the 100 is playing to an external device.

You could get a used S-2 compatible Connect instead of a Port, they are no longer sold new.