I vote for the airhorn. You cannot deny hearing an airhorn, even with earbuds. And it won't get drowned out by city noises, either.

I think that alot of people (not you, necessarily) think they are shouting "on your left" but it comes out alot softer, and way too late. I've been passed by people who are quietly saying on your left as they are passing me. That doesn't do any good.

I use a bell and it's a loud one (Incredibell) and it seems to really work. With a bell or airhorn, there's no language barrier to overcome, either.

You mention that you yell out, but did your friend yell out "On your left?"