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[aeharding]

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[alexanderbrevig]

Welcome!
After a rough glance it seems to me you'll need a bettery actually connected (and regulator enabled in order for you to use the barometric sensor. Is this intended?

Also, I would not cut out decoupling capactiros for the AVCC and VCC lines. Be sure to place capacitors close to the pins for best performance

[jwm_]

Have you checked the quiesent current of your buck-boost regulator/minimum load? Some can be quite voracious. I recently ran into one with a 20ma standby current independent of load. And to think I originally added it in to improve battery life, trading losing half a ma via a linear regulator for 20ma continuous draw. doh.

[ralphd]

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.

[microbug]

Step 0: use KiCad (or anything else but Eagle).

[obiwanjacobi]

Quick scan:

Only one terminal of SL1 is connected? Also the pad right next to it is unconnected.
There are a number of holes that seem to serve no purpose?

[2c]