Can you hook up an old surround sound to a new Smart TV?
You can hoop it up. Connect every speaker in the surround sound to the AV receiver with the help of wires.  Connect this AV receiver to the TV with HDMI ARC/eARC, and you have successfully connected the old surround sound to a new smart TV. If you have an old AV receiver, then connect the media source to the TV with the help of an HDMI cable and connect the Audio-out RCA ports of the TV to the audio-in RCA ports of the old AV receiver.  With this step, you have successfully connected an old av receiver to a new smart TV.