Conflict between Audio Amp Shim and Inky Phat?

Hello buccaneers!

I’ve got a bit of an odd one. I’m trying to attach a Button Shim, Audio Amp Shim, and Inky Phat on one poor little Raspberry Pi Zero. The Button Shim I’ve soldered onto the Pi’s underside, and I’ve attached the Audio Amp Shim followed by the Inky Phat on top. My header soldering wasn’t very neat because I had to solder from both directions with the Button Shim in place, but I’ve run a multimeter around the pins and everything seems to be making contact.

With all three components in place, the Inky Phat and Button Shim work great, but no sound comes out of the Audio Amp Shim. If I remove the Inky Phat, the Audio Amp Shim starts sounding immediately.* But also, if I remove the Inky Phat, the Button Shim stops working (returning Errno 121 Remote I/O error)! If I place the Inky Phat ever so gently on the top end of the header (pins 1-6 or so) until it makes contact, the sound cuts out. And the buttons start working. Pull it off, the sound starts again. And no more buttons! While fun, this is not ideal for the feasibility of my project.

Looking at Pimoroni and, it doesn’t look like there should be any conflicts between the Button Shim, the Amp and the Inky Phat. Does anyone have any idea what could be causing this, or can anyone replicate?

* I’m testing with speaker-test and mpg123.

Edit: Trusty Mr Piece of Wire and I have discovered that I can mute the Audio Shim if I short pins 12 and 9 or 12 and 14, i.e. GPIO18 to ground. This is expected since I think the Audio Shim uses GPIO18, but could this somehow be connected to the Inky Phat?