It is not uncommon for lower grade speaker wires to be copper plated soft iron core. However, it was shown that only a small portion of the cable would react to a magnet.
I have a solution. Folks may take it as they please.....
ECM (electrochemical migration). This effect is a very common problem in my industry.
Proof would be to remove the magnetic portion of the wire and repeat the experiments with one catch..
Keep the humidity of the room below 80%. ECM requires the following to happen:
1. Humidity above 80%
2. Certain atmosphere contaminants (ppm can be extremely small but the contaminants aid in ionization)
3. Voltage difference (poor connections
)
4. Heat (poor connections)
5. A metal to migrate (silvered, tinned soft steel like found in most passive components and terminals)
This is why iron seems to form at a carbon spark gap.
Insulated wires are extremely susceptible to this problem. The migration only travels a short distance from where the migrating metal is.
None of it can happen unless the humidity is fairly high. It can happen at lower humidity levels but the other factors need to increase to make it happen.
Like the heat, voltage, difference between the metals, etc.
An example is ribbon control cables. In the circumstances we deal with, ribbon cables must be replaced yearly because iron from the components on the circuit board leaches up the cables and causes the insulation to crack/open. All this with only spilt-rail 15V supplies.
Another example is multi-layer circuit boards shorting between traces in the middle layers because of ECM dendrite growth.
Another is high power DC choke failure in motor drives. Iron from the copper coated and silver plated terminals migrates along the windings of the choke causing a gradual increase in choke inductance.
In the old days it was called 'whiskers' when solder grew whiskers on a junction.
All unusual to see in the public except where folks twist wires together on high voltage connections and don't expect problems