refactor uses of voicings

+ fix usages of scaleTranspose

tutorial/tutorial.mdx

@@ -769,7 +769,7 @@ Together with layer, struct and voicings, this can be used to create a basic bac
 
 <MiniRepl
   tune={`"<C^7 A7b13 Dm7 G7>".layer(
-  x => x.voicings(['d3','g4']).struct("~ x").note(),
+  x => x.voicings('lefthand').struct("~ x").note(),
   x => x.rootNotes(2).note().s('sawtooth').cutoff(800)
 )`}
 />