Yay, Overtone works with a local copy of SuperCollider on Windows. [lib.clj](https://github.com/overtone/overtone/blob/master/src/overtone/helpers/lib.clj#L345) was the culprit!

commands.clj

;; commands to run in the REPL
(use 'overtone.core)
(boot-external-server)
(definst foo [] (saw 220))
(foo)
(kill foo)

;; voila!

lib.clj

(defn windows-sc-path
  "Returns a string representing the path for SuperCollider on Windows,
   or nil if not on Windows."
  []
  (when (windows-os?)
    (let [p-files-x86 (System/getenv "PROGRAMFILES(X86)")
          p-files-x86 (File. p-files-x86)
          p-files-x86 (map str (.listFiles p-files-x86))
          p-files     (System/getenv "PROGRAMFILES")
          p-files     (File. p-files)
          p-files     (map str (.listFiles p-files))
          parent-dir  (map str (.listFiles (File. "..")))
          joined      (concat p-files p-files-x86 parent-dir) ;; join the output of the 3 directories
          sc-files    (filter #(.contains % "SuperCollider") joined)
          recent-sc   (last (sort (seq sc-files)))]
      recent-sc)))