Many modern digital cards allow you to embed an MP3 file that plays as soon as she opens the link.
Use a video editing app to layer the song over photos of her from the past year.
Music anchors our memories. When you play a specific song every year, it becomes a tradition she will carry into adulthood. A high-quality MP3 download ensures you aren't reliant on spotty Wi-Fi or interrupted by ads during her big moment.