Volume is not why people stop and listen... but you can (and should) learn to play at a decently loud performance volume.
My band busks regularly on a corner with our backs to the street and facing a wide sidewalk with lots of foot traffic. But it means every time a loud car or motorcycle passes by behind us, we're drowned out for a few seconds. The band is a four piece and the instruments are DB, violin, guitar/bangos/mandolin and mandolin/bango/washboard. Everyone sings. We can play pretty loud and we never use amplification.
I find if what we're playing is interesting to people then a crowd will gather and listen. Sometimes 50 -75 people. Sometimes up close. If we're noodling or whatever they just keep walking and no amount of volume would make them stop.
You only have to play loud enough to be heard and then play something worth listening to for the intended audience, otherwise volume is just another way to annoy...
...my $.02
-Jeff
Fine, in the ideal sense and if your audience is up close but if you can't even hear yourself very well, it's really a drag.

