Noteblocks don't always act as the intended instrument when on top of a certain block.
As an example, all wooden trapdoors under a noteblock make the contrabass sound, but instead of making the iron xylophone (vibraphone) sound, the iron trapdoor is unrecoginzed by the noteblock & so defaults to the harp instrument.

The wooden/stone/blackstone pressure plates make the proper sound, as do their respective buttons, so many of these other transparent blocks should as well.