Brecht's script has a few songs written into it. Most notably, a scene near the end shows Courage and her daughter traveling in circles while an off-stage voice sings about longing for home. So the lyrics were already written by Brecht and the translator.
This isn't to say the play is a musical. It isn't even what we usually call a "play with music", except by the strictest definition. The first major production in NYC starred Anne Bancroft and was directed by Jerome Robbins. It had music by Paul Dessau, per IBDB.
Interestingly, when Brecht was asked who was his ideal Mother Courage, he replied, "Ethel Merman". But I think he was talking about Merman's indomitability, not implying the play required her famous singing voice. And, also, Brecht liked to say shocking things and perhaps shouldn't be taken too literally.
Apparently...when Brecht and his wife Helene Weigel were in New York in 1948, they attended several Broadway shows, including the original production of Annie Get Your Gun. Merman had a famous running gag every time she saw Frank Butler: She dropped her jaw and stared, as if she had never seen anything so beautiful.
If legends can be believed...Helene Weigel was so taken by Merman's long, slow, slack-jawed take that it became the basis for Mother Courage's famous "silent scream."
Weigel told producer Cheryl Crawford that Ethel Merman was the only "indestructible woman" she had ever met, and thus, the basis for Weigel's characterization.
By way of confirmation, Glenda Jackson also said that Lotte Lenya once told her that Weigel's nickname for Merman was "Mother Courage." According to Jackson, Weigel saw Mother Courage as having Merman's "big, blousy" quality.
In 1961, Cheryl Crawford and Jerome Robbins were (inexplicably) contemplating a Broadway production of Mother Courage, and they asked Weigel who should play the title role.
I don't know that Brecht or Weigel ever saw GYPSY, but if you think about it, Madame Rose isn't that far from Mother Courage. Both are driven women who can't see the forest for the trees, so to speak, and mothers who sacrifice their children while deluding themselves that they are "saving" the kids.
Thanks for reposting that, joey. The Merman remark has been attributed to Brecht by so many sources, I have no idea where it originated. Perhaps Weigel's remarks were attributed to Brecht by mistake, or perhaps they simply shared the same opinion. I'm sure Brecht loved the sheer presentationalism of Merman's performance, as he was no fan of American "method acting".
BTW, Crawford and Robbins contemplating MOTHER COURAGE may seem inexplicable, but they mounted their production in 1963, at the Martin Beck. What is inexplicable to me is that Anne Bancroft (though a great actress, IMO) was only 32 when she played the title role (a woman who has two grown sons)! Of course she was only 6 years older than Dustin Hoffman when they did THE GRADUATE, but I expect that from Hollywood.