I think of podfics as adaptations, which are generally a different category than the kinds of works that OTW and U.S. Copyright law have in mind viz. transformative use. I don't think it's necessary to claim podfics as transformative in order to value the artistry and craft that go into them, or how they can offer an interpretation of the fics.

But I don't necessarily consider transformativity as an aesthetic value in and of itself (vs. a legal concept). For instance, an AU story would typically be more transformative viz. the source then a casefile fic, and slash would be more transformative than gen, etc., but that doesn't mean that the former types of stories are inherently better or more -- interesting? creative? -- than the latter.