If you don’t feel like opening up iTunes on your Mac, you can always use Finder’s Quick Look feature to play a song. But you can not continue playing the song in the background. Spotlight Search will allow you to play a song directly and in the background.
Activate Spotlight Search using the usual keyboard shortcut Command + Spacebar. Type the name of the song to let Spotlight search your hard disk. Select the song from search result, hover over the album artwork and click on the play button.
You can now exit Spotlight Search and the track will continue to play in the background. To pause or stop the song, activate Spotlight and click on the same button on the cover art.