781.6 is traditions of music, 781.62 is folk music, and if you want to tie it to opossums I probably up it under "history and description of folk music with respect to kinds of persons; treatment with respect to specific racial, ethnic, national groups" -- i.e., Folk music of specific racial, ethnic, national groups: 781.621-.629. What race, ethnicity, or nationality is the opossum(s) you have in mind?

Or we could go at it from the opossum angle instead of the music angle: 599.23 for Australian possums; 599.232 for brush-tailed and scaly-tailed possums, including Phalanger (cuscuses); 599.27 for the shrew opossums, or 599.276 for Didelphidae (American possums). Then tack on a zero and add a standard subdivision for traditional music -- perhaps 78162 -- so the classification number for Didelphidae in music might be 599.276078162.

See? Nothing at all hard about it if you know the possum's background.

Now, flat possums on the highway in traditional music -- or, alternately, traditional music about road-killed possums, could open up the 390 area which deals with death and funerals....