/lib/udev/rules.d/90-alsa-restore.rules is in alsa-utils 1.0.25-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | ACTION=="add", SUBSYSTEM=="sound", KERNEL=="controlC*", KERNELS=="card*", \
TEST=="/usr/sbin/alsactl", RUN+="/usr/sbin/alsactl restore $attr{number}"
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