From our interview at 
www.gothic-rock.de :
Check out the Mp3's. 
Markus: And did you do some vocals on the Morningrise? 
     Dan: No. Thats another thing. Ive never done anything on any OPETH album. And
     you can torture me, (laughing) or burn me or whatever. 
http://home.tiscalinet.de/nachrodt/claret/dan_any.mp3
No! Mike was really, really
     way on the fact that I was not gonna do anything. I dont play anything. I dont do anything
     on any OPETH album. Our voices are really similar! Because we come from the different
     kind of school or what I can say ... we just use our voice in the same way, and if you would
     hear the mastertapes without the effects you would hear that it was different. And I think
     people must hear now, when hes singing clear all the time and compare it to Orchid or
     Morningrise that thats Mike, its not me.
Markus: Its only one part. That Survey the .... 
     Dan: I wrote some parts on Morningrise, I wrote some vocal harmonies. (Singing 

 Na,
     na in the forest unknown .. is some part 
http://home.tiscalinet.de/nachrodt/claret/dan_sing.mp3
that I actually .. I wrote the vocal harmony,
     but Mike sung it, so people might think: Oh, that sounds like he singing. Because
     it was my part. He was having problems finding the vocal harmony. I was just doing the
     way I did. I played things on like a flute sound on the keyboard and then, this could work
     and he just kind of sung it. That worked perfectly, but no, I  didnt do anything. Nothing.
     They didnt allow me to interfere with their magic (laughing).