Obviously I didn't use these "placeholders" in the original piece. Instead, if you have a GM compatible soundfont (Such as GeneralUser GS, FluidR3), you can replace those SF2player slots with the sounds instead (music box, slow strings, piano). I used SGM v2.01 (chopped into smaller SF2s, because the big chump of SF2 wouldn't work with LMMS).
It's awesome. I specially liked how it transits from one part into the other every time with a different approach.