The Songcast dialog lets you establish and terminate connections between the existing Songcast Senders or Receivers on your network.

Songcast Sender modules currently exist for Windows and Mac OS X.

upmpdcli also has a Songcast Sender additional package. Unlike the Mac and Windows tools, this does not capture arbitrary audio on the Linux host, but it is useful for synchronized multiroom audio.

Songcast Receiver modules come with Linn hardware, or with software Renderers like upmpdcli or MediaPlayer. As far as I can see (disclosure: I am the author of upmpdcli) the Songcast Receiver in Upmpdcli works much better at the moment (12-2015).

You will see Senders listed in the top part of the dialog if there are any currently active on the local network.

The bottom part of the dialog list the Receivers, with their connected state.

If a Sender is selected in the top part, you can check the Link to selected Sender boxes, and click Apply to establish the connection(s).

Checking Unlink and clicking Apply will disconnect the selected Receivers.

You will need to close and re-open the dialog to update the lists when Senders appear, or Receivers appear or disappear from the network,

The Senders are currently not too good at advertising their demise, so you may be better off restarting upplay after one goes away, to avoid timeouts when using the dialog.