Yes, you can connect an analogue synthesiser to the Sonos Port via the line-out or headphone jack as it supports analogue inputs. The port converts the analogue signal into a digital WLAN streaming signal, which you can then stream to the Sonos Arc Ultra.

However, streaming does not happen automatically as soon as a signal is present. You have to select the source in the Sonos app and set the port as the playback device so that the signal is transmitted to the Arc Ultra. The port is not designed to automatically recognise and start the stream without app interaction.