Here's the finished layout of the first NoiseCard:

The design is fairly straight-forward, no minimum trace widths or impedances to worry about. The digital microphone means little worry about analog noise too. Some notes:

• Dimensions: 85mm x 50mm is slightly less than a "standard" business card, but this really doesn't need that much space. This is more of a production specification, but I also opted for a 0.8mm board thickness. The thinner board was no extra cost, and it helps make the PCB more of a card.
• On-off switch on the left lets you hold the card with your hand away from microphone port.
• Decibel scale: larger spread below 80dB since (I believe) most people would find those levels to be tolerable. If we're measuring over 80dB, we're more interested in precision to know how loud (and damaging) the noise is.
• Healthy/dangerous notes: I wanted to provide info on the card so users can understand more about noise levels and their potential harms. With half of the card empty, I would have loved to fill it up with info, studies, facts, etc, but coming up with all of that would have delayed sending this PCB to production. I just left these "safe", "unhealthy", and "dangerous" remarks to start; in the future, I might print a sticker with more info that could go over the PCB.
• Top-left hole: Maybe you can throw this on a keychain?

Production

I ordered PCBs and assembly through JLCPCB. I had previously ordered personal stock of the microphone, so it made sense to use that up. For five boards, it cost $4 for the PCBs plus $50 for the assembly (minus the solar panel and 1.8V regulator which were unavailable); about $11 per card.

Next, we'll see how well these cards perform, and if the on-board solar power is enough to make them useful.