An Engine that Harnesses Sound Waves

I had never heard of Sterling engines before, so I found this quite fascinating.

I have a question though to whomever may offer an explanation. Towards the bottom of page 1, the article explains that "[w]hen pressurized helium on the top side of the metal plate is heated, sound waves traveling through the gas are amplified, causing the plate to vibrate...."

When the pressurized He is heated it causes the sound to be amplified? But heating the He should cause the density to decrease, of course, and therefore reduce the power of the sound waves propagating through the He, no?

I'm not aware of any relationship between frequency and amplitude, just V=(freq)(wavelength). What's going on? The only thing I can think of is that perhaps there's some sort of resonance going on in the chamber? As the pressure drops, the speed of sound should drop. But this shouldn't necessarily affect the frequency... so I'm totally confused. Maybe there's a resonance effect as the waves propagate slower and slower and interfere with one another? I'm totally lost. I just don't see how decreasing pressure (by increasing temperature) increases amplitude.

It's been quite a few years since studying E&M as an undergrad, so I'm a bit rusty and just genuinely curious what I'm missing here.

Thanks!