Even if you're not planning on having dancing, background music really helps set the atmosphere at the party. Make sure you have a range of fast, slow, soft, and quiet songs on hand to fit the mood at different times of the event and get your guests feeling celebratory.
If you're hosting a kids party, pick out some kid-friendly selections that won't drive the parents crazy. If you're hosting a grown-up celebration, make sure you have a range of genres available to suit everybody's likes, and let your guests play "DJ" too.