I remember reading a study several years ago that showed strong evidence that playing music in the store boosts sales—and even if you don't have an expensive sound system piping music in, you can purchase a relatively inexpensive CD player and speakers.
If you don't have one already, think about creating a register display centered around your holiday music CDs. Put out a sign near your register promoting the CD you're playing and offer a special discount with purchase, such as spend $30 or more in the store and receive $5 off the CD, or $3 off the CD that is currently being played. Suggestively sell to every customer who comes to the register by asking if they want to purchase a CD at the special price.
Don't be surprised if you hear your customers singing along to the music as they do their holiday shopping.