I hope they will fix this when the game is out. PM stated they would update the toolset.
Maybe, there is a way to manually add the sounds (like with the spreites for special attacks).

Update: There is a guide on a modding site about this topic, but it's in Korean.

https://gall.dcinside.com/mgallery/board/view/?id=lobotomycorporation&no=572520&search_head=110&page=4

引用自 hurricanelily.j.a：

Update: There is a guide on a modding site about this topic, but it's in Korean.

https://gall.dcinside.com/mgallery/board/view/?id=lobotomycorporation&no=572520&search_head=110&page=4

Pog

Korean or not, I hope we'll at least get to see some custom sounds on keypages from the workshop soon

This is 100% something I support and I was actually thinking of making a post about it myself. Even if it's just a selection of presets (which ideally would have as many as possible), something that let you use the game's diverse collection of sound effects for your Workshop Skins would be super cool. Hopefully they let us use as many of them as possible!