The images are all but public domain, yes; this didn't happen until after we'd started work porting. However, no, we don't have explicit permission for the music or sound effects. However, these are available freely as a download from Jason's personal site. While it would be nice to package the whole thing up without requiring other downloads, we'd need to talk to Jason and/or Alex Brandon about the remaining data files.
BTW, yes, the license is GPL. Also, the svn repository (using Google Code) allows anonymous read, so anyone who wants to try building it is welcome to do so. Can't promise it'll work for everyone, though.
I see by the replies that you want to keep the original port as close to the Pascal as possible, but after that could I help out with it? I'm much much more accustomed to C++, but I really want to help out on expanding this.
Also, I'm having trouble compiling this in Windows. Mainly getting MinGW's make to run the makefile. Ugh, I have these problems everytime I reformat... Anyway, so my current error is *** No rule to make target `obj/backgrnd.o', needed by `tyrian'. Stop. I'm guessing all the rest would follow suit (background.o is the first file in the makefile).