"On your left" is secret code, known only the initiated. Since there's no initiation required to use the bike path, it's silly to expect everyone to know what it means.
I'm in favor of plain English. "Bike passing" says it without confusing anyone.
That's if they can hear you over the iPod…
I slow down, prepare to brake if necessary, and yell "PASSING!". Almost always, they turn to look then move to the right. I'm most cautious when the walker has a dog on a long leash.