An opera waltz, like any portion of a larger musical work, is styled roman, in quotation marks.

The Chicago Manual of Style explains, “Titles of operas, oratorios, tone poems, and other long musical compositions are italicized and given standard title capitalization. Titles of songs and other shorter musical compositions are set in roman and enclosed in quotation marks, capitalized in the same way as poems” (“Operas”). This means that if the composition is referred to by its first line of text, its title will use sentence-style capitalization (“Poems”).

The sentence below refers to a waltz first by its opening line of text, using sentence-style capitalization, and then by its popular title, using title-style capitalization. The larger work containing the waltz, the opera La bohème, is italicized.

Act 2 of La bohème features one of the most famous selections from any opera, known as “Quando m’en vo” or “Musetta’s Waltz.” 

Introduction to Research in Music notes that terms identifying the genre of a musical selection “are left as lowercase words when they are not part of a specific title” (Wingell and Herzog 213).

The waltz from Eugene Onegin includes sung text but is sometimes recorded with instruments alone.

Compositions that belong to a specific genre (waltz, march, etc.) may also have titles that do not include the generic term. 

George Balanchine’s ballet Vienna Waltzes begins with Johann Strauss’s “Tales from the Vienna Woods.”

Works Cited

“Operas, Songs, and the Like.” The Chicago Manual of Style, 17th ed., sec. 8.194, U of Chicago P, 2017, www.chicagomanualofstyle.org/book/ed17/part2/
ch08/psec194.html.

“Poems Referred to by First Line.” The Chicago Manual of Style, 17th ed., sec. 8.182, U of Chicago P, 2017, www.chicagomanualofstyle.org/book/ed17/part2/
ch08/psec182.html.

Wingell, Richard J., and Silvia Herzog. Introduction to Research in Music. Prentice Hall, 2001.