Knowing when to walk down the aisle

I'm having a small beach wedding of 20 people. There is a walk from the parking lot, down the dunes to the beach where the ceremony is. I have a specific song I want played while I walk down the aisle. But what I'm wondering, is how am I supposed to know when the song starts playing and i start walking? I won't be able to hear the music from the top of the dunes (it's just being played on a small Bluetooth speaker). And there is no phone reception at the beach so I can't have anyone text me to let me know. Any advice?