I suggest a secondary release type "audio play".

Reasons:

• Currently audio plays are added to the secondary types audiobook, spokenword, both, or none. A special type would end that confusion.
• Audio plays are significantly different from audio books. In audio plays there is often no role as central as the narrator of audiobooks. There might be music, songs, sound effects which are not found in the majority of audiobooks.
• We did a similar thing for works, when we added the type "play".
• I chose "audio play" over "radio drama", "audio drama", "audio play", "radio play" or "radio theater" (see http://en.wikipedia.org/wiki/Radio_drama) because many audio plays are not "dramas" or not broadcast by "radio", and "audio play" should minimize the confusion.