Findings from independant book research during lesson:

The Cinema Book 2nd Edition
Edited by Pam Cook and Mike Bernink

The musical has always been a mongrel genre. in varying measures and combinations, music, song and dance have been the only essential ingredients. In consequence its history, both on stage and screen has been marked by numerous traditions, forms and styles. These in turn have been been marked by numerous terms- 'Opereta', 'revue', 'musical comedy', 'musical drama', the back-stage musical' , the rock musical, 'the integrated musical and so on.
... Historians, critics and theorists of the musical sometimes disagree about the meaning of some of these terms. ... some invent their own. .. it is still possible to provide some basic definitions, to indicate areas of debate and disagreement, and in the process to highlight the extent to which the musical has always been, despite its accessible and effortless image, a multifaced, hybrid and complex (Collins)

Musical Drama:
- Many of these started off as Broadway musicals.
-Large emphasis on story line
-Mueller - talks about the relevance of the music in terms of whether they advance the plot or not.
-There has remained a constant commitment to comedy and the comic, to popular and vernacular styles of music, song and dance, and a willingness to sacrifice coherence or integration for the sake of either or both.
(more to add... )

More sources for reference that i copied down from the book:

Rick Altman, Genre:The Musical, London, RKP,1981

Rick Altman, The American Film Musical, Bloomington, Indiana University Press, 1987

Jim Collins, 'The Musical' in Wes D. Gehring, Handbook of American Film Genres, New York, Greenswood Press 1988.

Jane Fever, The Hollywood Musical, London, Macmillann 1982 and 1993

Martin Rubin, Showstoppers: Busby Berkeley and the Tradition of spectacle, New York, Columbia Univeristy Press, 1993.

John Mueller, Astaire Dancing: The Musical Films, New York, Knopf, 1985