The sound reported by NEW SOUND isn't a variable of any kind; it's a value, of type Sound. You can do anything with it that you can do with values. I mean, for example, when you do (ROUND (3.14)) it reports a number, namely 3. That isn't a variable, but, like any other value, you can set a variable to it; you can put it in a list; you can SAY it; you can query its type with the IS _ A _ block; and so on. Same for sounds! You can set a variable to a sound; you can put it in a list; you can SAY it; etc.
I used to tell my SICP students, "Any time you find yourself wanting to ask 'Can you use X as input to Y?' the answer is always 'Yes.' " Data are data.