Yes, this is not only a problem in your blog (or your theme). It is a problem of podPress and it will be fixed in v8.8.5.
A patch for that problem is already in the current Development Version (8.8.5 beta 2).
If you try this beta version then be aware that there are still some eventually visible bugs. If you use especially 8.8.5 beta 2 then don’t disable the player via the option Post Content > Player on the General settings page of podPress. (It would scramble your blog appearance. Not permanently but as long as the player would be disabled this way. It will be fixed in 8.8.5 beta 3 of course.)
But I would like to discuss the m4a player preview in general. For video episodes you can choose a different cover image for each episode. But that is not possible for m4a files (at the moment) although the video player preview is displayed by default. If syou click on the preview the Quicktime browser plugin will be loaded in a predefined frame. But it seems to me that the Quicktime browser plugin does not show cover art which is embedded in a m4a file and because of that I’m thinking that the preview player is not a real preview. In conclusion: in my opinion it might be better if the preview of the m4a player is more like the appearance of the real player.
What did you think about that?
(podPress 8.8.5 beta 3 will probably have a different player preview for m4a files. But if you all disagree with my opinion then I will probably change that again. For the future it would be very nice to have a free flash player (or something comparable) which can play m4a, show chapter marks and cover art of m4a files. Please tell me if you know of such a player!)