The green color is not an oxide of Cu. There are 2 oxides of Cu, Cu2O and CuO. CuO is black and Cu2O is a reddish brown color.
Copper sulfate (CuSO4) typically does not form on audio cables. It needs the presence of sulfuric acid and an aqueous environment to form. Actually CuSO4 is white in its anhydrous form but turns blue when hydrated.
The green color on Cu is due to either the formation of hydrated Copper Carbonate Cu2CO3(OH)2 or the hydrated form of Copper Chloride CuCl2(2H2O). These would typically not form on audio cables because they require the presence of an acid. However Copper Chloride can form on cables that have PVC as their insulating jacket. PVC degrades on exposure to UV and/or heat and undergoes an autocatalytic dehydrohalogenation degradation process which produces hydrochloric acid (HCl). The HCl can react with CuO on the surface of the Cu conductor forming copper chloride (CuCl2). In the presence of moisture (H2O), the CuCl2 becomes hydrated to CuCl2 (2H2O) and produces the green color.
This green color was widely seen on Monster Cable back in the day and was due to a problem in their manufacturing process. Monster Cable used a PVC jacket and If the PVC jacket did not have the proper stabilizers, it can start to degrade quickly.