Dressed To Kill

Dressed to Kill, also known as Prelude to Murder (working title) and Sherlock Holmes and the Secret Code (in the UK), is the last of Sherlock Holmes (1939 film series) starring Basil Rathbone as Sherlock Holmes and Nigel Bruce as Doctor Watson.
Three cheap musical boxes (each one playing a subtly different version of "The Swagman"), manufactured in Dartmoor Prison, are sold at a local auction house. However, a criminal gang is determined to steal and recover all three, even if it means committing murder. Sherlock Holmes tries to recover the music boxes and crack the secret code contained in the tune before the gang can get what they want.