This file stores current audio configuration, specifically which device is selected and what volume each device has.

It is stored in $XDG_STATE_HOME/wireplumber/default-routes, usually ~/.local/state/wireplumber/default-routes.

An example file looks like this:

[default-routes]
alsa_card.pci-0000_00_1f.3:output:analog-output-headphones={"channelMap":["FL", "FR"], "latencyOffsetNsec":0, "mute":false, "channelVolumes":[0.008000, 0.008000]}
alsa_card.pci-0000_00_1f.3:output:analog-output-speaker={"channelMap":["FL", "FR"], "latencyOffsetNsec":0, "mute":false, "channelVolumes":[0.000000, 0.000000]}
alsa_card.pci-0000_00_1f.3:profile:output:analog-stereo+input:analog-stereo=["analog-output-headphones"]

The key names are system specific, referring to the hardware part of your audio devices.

The values are json objects.

The values channelVolumes property is the volume (from 0 to 1) taken to the third power. the value $0.008$corresponds to $\sqrt[3]{0.008} = 0.2 = 20\%$.