明白了，并且现在没有问题了，谢谢 :)
Reason is that "song" refers to the Bitstream Cyberbit fonts.
And I cannot package the fonts, because it is not DFSG-free.
Use
\begin{CJK}{UTF8}{gbsn}
and it should work alright, because both "世" and "界" exist in
simplified Chinese.
-- lish