"part of a unit" just means that you could use songs in addition to other things in a particular part of your class. In a "general science" class you might have a "unit" on linguistics, and another on biology, and another on physics, for example.
Is that what you mean?
When I first read this I thought you meant it was some kind of technical usage, but after reading it again I realize it's just a description of lesson plans for a topic: play some songs, show some videos, have a discussion, assign homework, give a test, etc.