Yes, 555 is pretty bad. Even a CMOS device like the TLC555 consumes up to 400\$\mu\$A. I blame the resistor divider, the other parts can easily be made in the 1-10\$\mu\$A range.
If I understand your problem correctly you want to monitor a varying voltage and get a signal when it reaches a certain level, and that at low power; I presume because it has to run a long time on a battery.
You don't want a 555, less a microcontroller. You just want a low power comparator. The LPV521 is a Nanopower opamp, requiring 400nA maximum at 5V. No need to switch it on and off. Just apply the voltage to be monitored and the reference voltage to the inputs, and switch a MOSFET which in turn controls the buzzer. Apply positive feedback for the opamp to get a hysteresis to avoid oscillating of the output when the input voltage is around the threshold.
The circuit should consume less than 1\$\mu\$A, so that it can run for several years on a CR2032 button cell.
Note that to achieve this extremely low power the opamp has a very low bandwidth of 6.2kHz. Here you signal is DC, but in other applications it may matter.