Had a chance to look at some code!
It took me a bit to get things set up since I didn't have any Derelict stuff, so I just added the source files directly instead of messing around with building static libraries. The ones I used were
https://github.com/DerelictOrg/DerelictAL and
https://github.com/DerelictOrg/DerelictUtil, which could be newer than what you are using since all of the OpenAL stuff is in a single file now as opposed to being in derelict.openal.al, derelict.openal.types, and derelict.openal.functions.
Any who, the only changes I made to your files were in soundactor.d, where I commented out importing derelict.openal.types and derelict.openal.functions, and then I uncommented lines 105:
sound.setBuffer(buffer);
and 107:
label.setString(to!(dstring)(temp[(temp.lastIndexOf('\\') + 1)..$ - 4].dup));
Once I added libsndfile-1.dll into the bin folder, I ran it, selected a file, and then hit the play button....and well, it worked. Granted, I did get some runtime memory errors on occasion which are probably related to the sound class(I know how I'll fix them), but it loaded the sound file(I used both a .wav and a .ogg) and played it. It would only play a single file each time I ran the program, though. It worked in both debug and release configurations. I used my own dsfml-xxx-2.lib files, but I used the included dll's.
So...yeah.. I really wish I could help but I don't know what I could do. Unless there was more that I needed to uncomment to see the error, it sounds like it is something in your own code.