Do you remember the "Little Nicky" movie with Adam Sandler?
when he put a Chicago record and spinned it backwards...
Back at the good old days I tried to experiment with "reverse speech" after seeing some dude talking about it and showing examples like this on "Unscrewed" with Martin Sargent on Tech TV.
appearently these are not subliminal messages but the brain slipping up the rest of the info.Basicly when you talk, your brain forms the messages a certain way and every time you pronounce a word or a sentence, you pronounce it a bit differently, therefore when recorded and run backwards it the same sentence might sound like another message backwards or it might sound like giberish.
I've actaully tried to look for these kind of messages in songs in different languages and found some cool stuff.I even tried to read text from a page and ran in backwards and got some cool results.
I once tried it on a few songs of an Israeli band singing in english and got reverse message in hebrew,so if we'll look into some opeth songs, we''ll probably find some messages in english and swedish.