"In the Mood" was one of the songs I listened to when I was working on a World War II project. Several of the other songs I grew fond of while researching and writing that book were "Praise the Lord and Pass the Ammunition," "Polka Dots and Moonbeams," "We'll Meet Again," and "The Last Call for Love."
Listening to music being played during the period I was writing about not only helped to put me in the mood to write, it also gave me a connection to the people who lived then. I could imagine how the words to a song like "Don't Sit Under the Apple Tree," reminded a young woman of the promises her soldier boy had made to her before shipping off. And, the ache she must have felt was, for me, nearly tangible.
You can find music all over the internet--I'm sure I don't have to tell you about YouTube!
The Internet Archive's Audio Archive is a treasure trove with hundreds of thousands of items!Duke University has a vast collection of sheet music that'll take you back to the early 1900s (and further) if that's the era you're writing about. Or, go to your local public library and browse through its collection of CDs. Look at the music section for books covering the time period you are writing about, even if you can't read music, you can get a feeling for the time by reading the lyrics and absorbing the language. In the library I work in, we have books such as Civil War Songs, and Singing the New Nation: How Music Shaped the Confederacy, 1861-1865 that would be useful with a Civil War project.