Whatsapp on iOS, how to delete voice messages?
Now, after 1000 views on this question it's worth updating my answer:

Recently, a function has been added to Whatsapp that lets you delete ALL voice messages from a chat.


open Whatsapp
hit "Settings"
click "Data and Storage Usage"
click "Storage Usage"
select a chat
click "Manage..."
remove every checkmark except for "Voice Messages"


(You can also delete other stuff in there but I wanna keep all of that haha)