The meaning group of a sentence (that is, words expressing the same meaning) is [flute], (sound) [melodious]
Both "flute sound" and "wandering melody" modify "sound".
This sentence is composed of [sound], followed by two meaning groups, one is [flute] and the other is [melodious].
And because [Wanyuan melody] is divided by [pause], flute and Wanyuan melody can only be separated by [comma].
Well, I hope you understand.