/lib/udev/rules.d/60-persistent-alsa.rules is in udev 204-5ubuntu20.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | # do not edit this file, it will be overwritten on update
ACTION=="remove", GOTO="persistent_alsa_end"
SUBSYSTEM!="sound", GOTO="persistent_alsa_end"
KERNEL!="controlC[0-9]*", GOTO="persistent_alsa_end"
SUBSYSTEMS=="usb", ENV{ID_MODEL}=="", IMPORT{builtin}="usb_id"
ENV{ID_SERIAL}=="?*", ENV{ID_USB_INTERFACE_NUM}=="?*", SYMLINK+="snd/by-id/$env{ID_BUS}-$env{ID_SERIAL}-$env{ID_USB_INTERFACE_NUM}"
ENV{ID_SERIAL}=="?*", ENV{ID_USB_INTERFACE_NUM}=="", SYMLINK+="snd/by-id/$env{ID_BUS}-$env{ID_SERIAL}"
IMPORT{builtin}="path_id"
ENV{ID_PATH}=="?*", SYMLINK+="snd/by-path/$env{ID_PATH}"
LABEL="persistent_alsa_end"
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