Honestly, aside from Melody 5 and Voice 4 sounding really dissonant and out of place, just about every other sound in the game's really well made. Especially the actual vocals. Fixing those two sounds to be more "in key" I suppose (if that's the right terminology) would likely make them both match the rest of the sounds but also sounds better.
This mod is quite interesting, just playing the effects by themselves sounds great. The effects provide most of the syncopation, which really improves the mix imo (also amogus)
In my opinion, I think even voices from younger people can work, but V4 feels like there wasn’t much planning on how it would fit. Any voice can work, but if there’s issues that would also affect voices by older people, it’s still going to cause issues. It doesn’t help the voice fit if it’s not matching pitch or having issues with volume. It can cause clashing with pitch or sound like it’s fighting for attention with volume. In the case for V4, like I said before, it just sounds like no planning was put into it and instead just dropped in. It just sounds really off pitch. Something like that could work, but it needs to make sense or it’s just going to sound weird.