I doubt the core audio format is different because I've had a similar problem with the 360 version which uses the same audio as the PS3 version except a majority of those files don't convert. Isn't WEM Wwise's proprietary audio format that must be converted to before used in current Wwise? I find it hard to believe the developers would switch audio formats for some of the audio in the same archive. I'll upload some examples from the unconvertible 360 WEMs in comparison to the PS3 version when I reclaim those files.
I've checked and can confirm that some of the WEMs are actually XMAs (Only tracks with ambiance or SFX though) so I guess that'll make it easier to sort.
However the same WEMs (from my previous uploaded pack) from both versions of the demo do not get the same error.