Say I write an article about some topic. Included in this article are a few fragemnts of news braodcasts, both tv braodcasts and radio broadcasts.

It is unclear how these should be marked up. I could makr them up as VideoObject and AudioObject, or I could mark them up as TVClip and RadioClip.

A TVClip or RadioClip van have a property "video" or "audio" with repsctivly the itemtype "VideoObject"and "AudioObject", but this makes little semantic sense.

Both the video and audio fragments are presented with a caption (which is accounted for in VideoObject but not in AudioObject).

furthermore I cann not mark up encodings for the available sources, as they are already marked up with the itemprop contentUrl:

<source itemprop="contentUrl" src="proxy.php?url=horse.ogg" type="audio/ogg">
<source itemprop="contentUrl" src="proxy.php?url=horse.mp3" type="audio/mpeg">
<source itemprop="contentUrl" src="proxy.php?url=horse.wav" type="audio/wav">

Just vending some steam here:
The Schema.org vocab seems like a total mess to me. I don't like it. But I hope I can assist in making it better, rather than leaving it be. For an organisation trying to mark up their website things like these are reasons not to mark up any further, the vocab has just not matured enough at this time (in my opinion).