We've been grappling with the same.  We've come up with several scenarios, depending whether one wants to pursue item level description, description of groups of items by media type, or description of intellectual components, regardless of media.  
You may recall a thread several months ago on the EAD ListServ about EAD for AV.  Our AV archivist Megan McShea collected a lot of examples from archivists across the country and set up an EAD for AV Google Group at 

Barbara D. Aikens 
Chief, Collections Processing
Archives of American Art, Smithsonian Institution
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-----Original Message-----
From: Encoded Archival Description List [mailto:[log in to unmask]] On Behalf Of John Harrison
Sent: Tuesday, December 07, 2010 12:29 PM
To: [log in to unmask]
Subject: Re: phys-what?

How about:

<physdesc><extent>1</extent> <physfacet>Scotch Umatic UCA 60</physfacet>


On Tue, 2010-12-07 at 16:43 +0000, Michele R Combs wrote:
> What's the appropriate element combination to describe the specific type of audio or videorecording, e.g. Scotch Umatic UCA 60?
> <physdesc><extent>1 Scotch Umatic UCA 60 tape</extent></physdesc>
> <physdesc><genreform>Scotch Umatic UCA 60</genreform></physdesc>
> <phystech><p>Scotch Umatic UCA 60</p></phystech>
> <physdesc><physfacet type="format">Scotch Umatic UCA 60 videotape</physfacet></physdesc>
> These all seem about equally right to me.  Thoughts?

> ~~~~~~~~~~~~~~~~~~ 
              '.    ,'.         John Harrison
             '  `  '  '         University of Liverpool
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