avoid resetting my whole library and re-importing.
I don't know why people ever consider re-importing a whole library. Everything can be fixed. Sometimes it takes a little longer, but there is learning along the way. The risk of throwing the baby out with the bathwater is just too high. It's just like re-installing Windows because of some minor issue. A targeted approach is far better and far less risky.
The bottom line in this case, it is highly likely that there is something that still has the unwanted value assigned to the field. If you create a View that shows all items, not just Audio items, and then search for the value, you may find the troublesome records in the library. That is easy enough. Just copy one of the Audio Views, perhaps the "Files" View would be best, and then uncheck the "Use parent scheme rules for file display" setting in View configuration to ensure all items are included. When you uncheck that setting you will see the list of accepted file types change in the Files Panes View.
Search using [field]=value, or if you want an exact match, [field]=[value].
My understanding is that those drop-down lists are created on the fly, and that there isn't any separate list of currently used values anywhere. I haven't tried to confirm that, but if correct, it is not possible for such a list to become out of date, or unsynchronised. Hence, the values must be recorded against a library item somewhere.