> there were still some functions and symbols that weren't acknowledged such as "display-musicxml" and "'atonal" (inside a def-score)
I am not a Opusmodus user (Just a Common Lisper) but IMHO, I think there is Common Lisp PACKAGE and SYMBOL related things like a described below link.
http://www.gigamonkeys.com/book/programming-in-the-large-packages-and-symbols.html
If "display-musicxml" and "atonal" symbols are not exported, You may IMPORT them, like
(defpackage :threads-test-pkg
(:use :cl :bordeaux-threads :opusmodus)
(:import-from ??? :display-musicxml :atonal))
If "display-musicxml" and "atonal" symbols are exported, You may USE-PACKAGE, like
(defpackage :threads-test-pkg
(:use :cl :bordeaux-threads :opusmodus ???))
;; How to find the ??? package => (find-all-symbols (string '#:display-musicxml))
-- g000001
https://github.com/g000001