There are a lot of runners on mixed use paths wearing headphones these days. They are an absolute danger to overtaking bikes. A bell they would hear would be useful.
I suppose you feel similarly about the dangers bikes pose to cars?
Pedestrians are not danger, they are victims! Cyclist should slow down, while performing dangerous overtaking, and not crash into them! Same rules like with cars!
To me, in a path with no priority to the bike, the only danger are cyclist who think they have priority and can overtake people at speed.
Being able to get the attention of runners improves the situation, reducing the speed while circulating on a mixed path solves it completely. If you wanna go fast get on a bike lane or the road.