I'd go with a 662k ldo to reduce board space and save cost. If the barometic sensor can take 4.2V then drop the regulator. Or run directly from a LiFePO4 battery.
Try driving the piezo directly; you may get enough volume and then can drop the transistor.
If your code size is not large you could save a bit by using an ATmega8a.
Is the 10-pin header ICSP? The 6-pin version would save some more board space.
Lastly, if the barometric sensor needs less than 5mA, you could run everything off a CR2032.