On multi-use trails I personally believe if you're going faster than someone else it's your responsibility to figure out the passing maneuver and it's your fault if there's any collision. If I see walkers with ipods I slow to nearly their speed and pass very casually. There's no need to be in a hurry to blow past people. If you are in a hurry, take the roads. Think of it as great exercise to accelerate back to your preferred speed each time.
+1 When I have to ride the MUP I slow down and try in a normal voice "on your left" which works about 75% of the time with an acknowledgement. If it doesn't slowly ease around them with a "good morning or afternoon." When there is a bunch moseying down the path it is with a not too loud of a voice "bike back" warning. If that doesn't work then a very resounding "bike back." never fails.
Just me. I hate bells. I believe they should only be used when there is a imminent danger, otherwise be civil and use your voice.