Drive the pose bone custom property.
Please note, didn't see the image correctly, it appears you have baked sound onto armature bone (the structure). In most cases recommend using the pose bones, which belong to the rig object, not the rig. If you really want to use the armature bones, go to second part of answer
Check out examples here https://docs.blender.org/manual/en/latest/animation/drivers/workflow_examples.html#workflow-examples
Without checking the tute, you are going down the garden path. Am assuming this custom property is on a pose bone of the object (pose mode), not a bone of the objects armature data (edit mode).
In example below have a property "prop" on a pose bone named "Bone". This was set in pose mode, with bone selected in the Custom Properties Panel with Bone tab selected. Consider this the prop I have baked sound on. Change names to suit your case
The object of interest is
which btw is
In driver variables we do not use this as the data path instead choose the ID object, be it an Object or an Armature, a Mesh etc etc.. It is an Object type in this case.
For a single property type driver variable the path is relative to object selected as the driver variable target. Hence in case above this is
and the path to the custom property from here is
This can be copied from the UI by right clicking over property and choosing "Copy Data Path" and pasting into path field of driver. Result of paste.
Using the Armature bone
On closer inspection it appears you have driven the armature bone. Would suggest changing this to the pose bone for most use cases.
If it is the case, then select the armature from Armatures when creating driver variable and use
as the path. As can be copied from
in object mode.