Rhythm on the River (1940), 6

Popular songwriter Oliver Courtney has been getting by for years using one ghost writer for his music and another for his lyrics. When both writers meet at an inn, they fall in love and ... 
1h 32min | Comedy, Musical | 6 September 1940
Director: Victor Schertzinger
Stars: Bing Crosby, Mary Martin, Basil Rathbone, Oscar Levant

http://www.imdb.com/title/tt0032986/

Oscar Levant's (b. 1906) 3rd of 14 acting credits, but the 1st two were in 1929. His performance is as you expect from him.

Lots of songs sung. According to Soundtracks page, MM gets 3 solos, BC gets 4, and they have 1 brief duet reprising a BC song at the end. It was very strange seeing both onscreen, 1 singing, and the other not joining in.

Although I've seen him as a composer in Make a Wish (1937), it's strange seeing BR in evening clothes and suits. I expect him either to buckle a swash, or don a deerstalker.

MM's 3rd film, although in the first she only provided a singing voice. She's been on Broadway by now, but seems a little subdued in this movie. Then again, she's playing a lyricist, not a performer.

The film illustrates nicely the problem of a ghostwriter who wants to go public, particularly a composer. When BC tries to get his own song published, he's accused of copying the style of his former employer.

Paramount, dir. Schertzinger; 6