get a bell. ring it. it works better than a voice because the walker just hears the bell, does not have to think about left or right. Most bikers who pass me who say "On your left" are already next to me, with no warning. I'm not slow on my bike, so those bikers are going very fast. If they come up on a ped, there is no warning whatsoever.

Get over your fear of having a bell on your bike. It works.