When I use amarok to open a file manager, konqueror is called instead of dolphin. In KDE5's System Settings, dolphin is set as the default file manager, as it was once upon a time in KDE4's System Settings.
I expect (and of course ICBW) the problem is that somehow my KDE4 settings got clobbered or otherwise screwed up and that amarok is looking to KDE4 settings. I no longer have KDE4's System Settings UI and grepping through ~/.kde4/* for about 30 minutes has not enlightened me any. I also grepped though ~/.config/* , where the KDE5 settings live, in hope that I might find something analogous to what I need to do for KDE4, but found nothing useful. (I grepped for 'dolphin', 'konqueror', 'fileman', and 'default' and will be happy to post results if anyone thinks that might help.) Searching the web for help has only turned up instructions for using System Settings to change the default. Any suggestions about where/what to search on my machine or the web are welcome. Of course, if you've got a handy recipe to post to fix my issue, that's even more welcome. :)