When skipPostRenderGui is true, the completion sound effect is also not played. I never upload from Minecraft and don't use explorer so the upload/open folder screen isn't useful for me. The complete sound is tho.

(Not sure how to add enhancement label)

Yesterday I noticed that the sound played on my headset but I'm pretty sure other sounds played through my speakers.
Today I wasn't wearing my headset, MC sounds definitely coming from my speakers but no completion sfx.
I'll try to wear my headset again next time and update.

Confirmed, Minecraft sound plays on my default device (being speakers) but the sound effect plays on my headset.