I second Jonathan’s comments about a bell.
I’m a big fan of the bell for several reasons. I good bell can indicate how far back you are and give an idea of how fast you are going. I like to ring it way back and then again when I’m nearer. A bell also has a better chance of getting through people’s headphones than your voice. A bell has a happy tone that can’t be confused for rudeness like say when you’ve said, “On your left” dozens of times already and may come across rude to people even when you don’t mean it.
Announcing your presence is quite important. I once very narrowly missed what could have been a serious accident when a jogger decided to u-turn right into me as I was passing him. Apparently, the bell wasn’t loud enough to get through his headphones. Also, pedestrians may do silly things as you pass them that could result in their or your injury. It’s best that they know you are there. Regular joggers along my route, flash a “thanks” sign with their hand when they hear my bell. It’s a nice confirmation that they know I’m there. Likewise passing another cyclist. I personally don’t like being passed without notice and appreciate knowing another cyclist is passing.