When passing walkers or runners on a MUP I simply call out "passing". In the USA, everyone understands you pass on the left. Also, I leave it to the pedestrian to decide what side they want to go on anyway. But solo walkers always go to the right.

My own experiences with someone calling out "on your left" is it usually sounds something like "uhm yub uft" and is said about a nanosecond before they pass. Whether or not someone understands that I said "passing", they certainly heard someone say something and know I'm coming. I call out well before I get to them and slow if needed until they react. Though sometimes there is no reaction and I can see ear buds in their ears. I make as wide a pass as possible and slow as needed. But it's on them if they get startled.

I tried a bell once, and I don't know what it was about that bell, though it sounded very typical, but people would start jumping all over the place. Like the bell sent a message of "get the F out of my way, now!"

I have also noticed the experienced walkers and runners. They stay far right, don't wander back and forth, and when I call out, they lift a hand or just a finger to let me know I heard them.

Worst is a small group of people that walk side by side across the whole MUP. I call out "passing" (usually multiple times getting louder until they notice), and do they all just move to one side. Of course not, some go left some go right and you have to thread the needle between them.

Plenty of cyclists coming the other way that think if their tires are just to the side of the middle, that they are on their side. They often make no effort at all to move over. I'm not dumb enough to play chicken, but I sure feel like just staying in the middle of my side and bracing for impact sometimes.