after editing the script a bit to omit the warning:
hexapod@hexapod:~/seeed-voicecard $ sudo ./install.sh
Hit:1 ht tp://deb.debian.org/debian bookworm InRelease
Hit:2 ht tp://archive.raspberrypi.com/debian bookworm InRelease
Hit:3 ht tp://deb.debian.org/debian-security bookworm-security InRelease
Hit:4 ht tp://deb.debian.org/debian bookworm-updates InRelease
Reading package lists… Done
Building dependency tree… Done
Reading state information… Done
All packages are up to date.
Reading package lists… Done
Building dependency tree… Done
Reading state information… Done
raspberrypi-kernel-headers is already the newest version (1:1.20230405-1).
raspberrypi-kernel is already the newest version (1:1.20230405-1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Reading package lists… Done
Building dependency tree… Done
Reading state information… Done
dkms is already the newest version (3.0.10-8+deb12u1).
git is already the newest version (1:2.39.5-0+deb12u1).
i2c-tools is already the newest version (4.3-2+b3).
libasound2-plugins is already the newest version (1.2.7.1-1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Creating symlink /var/lib/dkms/seeed-voicecard/0.3/source → /usr/src/seeed-voicecard-0.3
Sign command: /lib/modules/6.6.47+rpt-rpi-v8/build/scripts/sign-file
Signing key: /var/lib/dkms/mok.key
Public certificate (MOK): /var/lib/dkms/mok.pub
Certificate or key are missing, generating self signed certificate for MOK…