I think that is one thing the samples that come with DarkGDK are missing, is something that shows some basic use of the inbuilt Sound and Music functions... although they are pretty intuitive to use, a quick specific sample project wouldnt hurt.... maybe in a future release ?? (hint hint TGC

)
In the meantime I can only reccomend that you play around with the "Music" and "Sound" functions that are documented in the GDK, their actual names and return types describe very well what the functions do, along with the GDK docs.. but adding music is one of the easier tasks, esentially, you load it, set its volume etc and then tell it to either play through once or start looping through, you then check the state of it to make sure its doing what you want(still playing/looping etc), you can then allow it to be turned on/off with input or after a time has passed or when some condition is met etc...
If it ain't broke.... DONT FIX IT !!!