Why does the buzzer in this circuit have a voltage drop of 0.7V?
You don't say what kind of buzzer it is, but if I had to guess then I suspect it is a three-tab piezo coupled with a simple BJT circuit like this:







simulate this circuit &ndash; Schematic created using CircuitLab

This might account for your multimeter measurements and also account for the relatively low voltage measurement across it when active through a voltage source and added resistor. It's really hard to say for sure, though.

Have you considered the idea of opening things up and looking to see?