Thanks. I was afraid that would probably be the answer.The beeper on the Altus Metrum devices is connected directly to the GPIO of the micro-controller therefore and most likely driven with a square wave.
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There is no way to simply crank up the volume. This would require a redesign of the altimeter. But you can try to find a frequency that you can hear better. The frequency response of the beeper is a good starting point for which frequencies to try, but the end result also depends on your individual hearing and other factors.
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Source: https://product.tdk.com/info/en/catalog/datasheets/piezoelectronic_buzzer_ps_en.pdf (page 4).
Maybe there is a company that offers a louder beeper in the same form factor, but this would require soldering too. Besides that, tinkering with the frequency is probably the only thing that can be done.