Is there a difference in translation and meaning between these two sentences or are they practically interchangeable?
My translation: Please can you lower the guitar volume.
The verbs する and なる are a transitive-intransitive pair. When they follow the 〜く form of adjectives, they're kind of like "make" and "become":
Ａが 赤くなる "A becomes red" (A turns red, blushes, etc.) Ａが Bを 赤くする "A makes B red"
The main difference here is that with なる the subject turns red, while with する the subject turns the object red. And なる doesn't take an object at all!
Keep in mind that in a real sentence, が might be replaced with は, or the subject might be left out entirely, etc. This is just a simplified example to show how this grammar pattern works.
Now let's look at the guitar example:
Since this is imperative in form, the implied subject is the listener. The object is explicit. The speaker is asking the listener to please turn the volume down on the guitar.
Here, we have an object marked with を, but our main verb なる doesn't take an object, so this sentence is ungrammatical. Of course, the listener can probably figure out what you meant to say, but it doesn't really make sense the way it's written. The phrase 小さくなってください seems to say "please become smaller", and it doesn't take an object marked with を.