I have a big bell which makes a very nice sound.

If a pedestrian acts surprised, I say 'I just wanted you to be aware that I am passing you'. Half the people thank me and I hear them as I go past. If anybody gives me the finger, I don't know, I'm already past.

I also have an air-horn (compressed air can) strapped on the handlebar (this is for the pesky dogs that the owners let them loose on unsuspecting passers-by... 120 dB has chased away various size dogs). I haven't used it on humans (yet).

Safe passing on a narrow MUP can be an intricate affair, if the other party has earbuds on at high volume. I was stuck once behind a guy on a pedestrian bridge (maybe 5 ft), with earbuds on and doing lunges! We must have been a pretty funny sight, as I had to follow him about 70 ft, until the exit... I didn't have the air-horn then