Okay, it occurs to me that I've been simplistic thinking that the main abstraction point in communication is time: that is, that the main difference between talking/listening and any other media is that the message is delayed before being passed on, by recording a voice or writing something down. This is wrong.
There's a whole spectrum of abstractions, of break points, to be made, across time and many other dimension slices. With books, the abstraction is that they're really easy to copy and recopy. However, if the text is a message reference, it's referencing an immutable message. A website is a mutable message that maintains the same reference. That's a different slice. A radio programme is broadcast once and appears in multiple places simultaneously, that's a slice in a different place.
But trying to model this (because a description of what is is maximally complex and no better than looking at the thing itself. A model can be analysed): Consider a message sent between my mind and your mind. A break point is where this message can be handled, changed and referenced. One way of looking at this is how far from the source the message is changable. So a tv programme is alterable up to broadcast so it can have many people involved in creating it. A web page is alterable to the time of download. The mechanisms used for these different abstraction levels vary: writing, recording, using a message reference. But these are all tools to maintain control of a message for different lengths of its transit. That's a possible model.