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A Connect is a pure computing device, so practically all of the electrical energy consumed is converted to heat. So that's a 5.6W heat source based on known idle consumption*.
The Connect:Amp drives speakers. It has been measured whilst playing at ca. 8.9W**, which is a small increment on the above; seems reasonable given the typical efficiency of solid-state electronics. That's a 2012 measurement but let's assume it won't have shifted magnitude in recent hardware versions. But let's also assume that was a quiet playback and not driving the speakers much. Now Sonos claim it has a Class-D amplifier driving up to 55W per channel. Class D amps are expected to have efficiency of 90-100%, let's assume the worst case; that's a loss to heat of 2x55W * 10% = 11W at full blast.
So the worst case scenario for cooling purposes appears to be that a Connect:Amp is potentially a ~20W heat source, or 72BTU/hr.
* https://support.sonos.com/s/article/256
** http://danonit.blogspot.com/2012/01/how-much-does-sonos-connectamp.html
Thank you for taking the time to write this. It not only answers my question, but also provides a better understanding of the components.