that will return a new MyID3 object.
File dst = new MyID3();
This however, will return what the write() method returns. In this case void. (I presume)
File dst = new MyID3().write(src, dst, src_set, metadata);
To fix it, do this:
File dst = new MyID3();
dst.write(src, dst, src_set, metadata);
And of course, the same rule applies to this line:
MusicMetadataSet src_set = new MyID3().read(listOfFiles[i]);