Scores from composer collected edition sets will have a call number on the spine beginning with M3. To cite a score from such a set, follow the rules for citing a volume from a multivolume work. See Chicago 14.21
When gathering the information you need for the citation, check to see if there are editor(s) both for the overall multivolume work as well as the individual volume/score. Your citation will have two titles that are italicized: the multivolume set title and the title of the individual volume/score you are using.
Examples of Bibliographic style citations:
Verdi, Giuseppe. Il Trovatore. Libretto by Salvadore Cammarano. Edited by David Lawton. Series 1, vols. 18a. and 18b., Works of Giuseppe Verdi, edited by Philip Gossett. University of Chicago Press, 1992.
For this collection there are two editors to include: one for the individual volume/score and one for the multivolume work.
Schütz, Heinrich. Die evangelischen Historien und die Sieben Worte Jesu Christi. Vol. 1 of Heinrich Schütz sämtliche Werke, edited by Philipp Spitta. Breitkopf & Härtel, 1968.
For this collection there is only one editor to include: the editor for the overall work.
Schütz, Heinrich. Cantiones sacrae (1625). Edited by Heide Volckmar-Waschk. Vols. 8 and 9 of Neue Ausgabe sämtlicher Werke. Barenreiter, 2004.
For this collection there is only one editor to include: the editor for the individual volume/score.
Mozart, Wolfgang Amadeus. Sinfonie in C, K. 551. Edited by H.C. Robbins Landon. In Orchesterwerke, series 4, vol. 9 of Neue Ausgabe sämtlicher Werke. Barenreiter, 1957.
In this multivolume work, the volume (Orchesterwerke) contains more than one symphony. To cite an individual symphony from the volume, we proceed as though we are citing a chapter from a book. The individual symphony appears in the title field, then the volume title (Orchesterwerke) follows after the word "In."
Mendelssohn Bartholdy, Felix. Lieder für Männerstimmen. Edited by Wolfgang Goldhan and Ralf Wehner. Series 7, vol. 4 of Leipziger Ausgabe der Werke von Felix Mendelssohn Bartholdy. Breitkopf & Härtel, 2013.
In this multivolume work, the title page states that the Saxon Academy of Sciences in Leipzig edited the collection, but usually corporate entities and institutions are not listed as editors in a citation. Corporate entities can appear as authors, but only humans appear as editors. Thus, only the volume's (human) editors appear in the citation.
Bach, Carl Philipp Emanuel. Vier kleine Duetten für zwei Claviere. In Miscellaneous Keyboard Works II, series 1, vol. 8, bk 2 of Carl Philipp Emanuel Bach: The Complete Works. The Packard Humanities Institute, 2005.
Sometimes a volume is split into two or more parts, books, or Teilbands. If you are citing an individual work from such a volume, you will need to indicate which part/book/Teilband contains your work. Use the abbreviation "bk." and the appropriate number after you give the volume. This information is not needed if you are not focused on an individual work in such a volume. See Chicago 14.23, "One Volume in Two or More Books."