Hi,
Sonic Visualiser uses libsndfile (
http://www.mega-nerd.com/libsndfile/) for importing uncompressed audio formats, so it should support any format listed at that page. Because this is a third-party library, it is unlikely that we can do anything about it if your file is not supported -- you would need to convert it to something that is.
I'm afraid I don't know anything about the format(s) used by Dalet, so I can't comment any further without more information about what the file format actually is, whether it is one that would be expected to be supported by libsndfile, and what error you are seeing when the file fails to load.
Chris