The label on the back of your CD player says to use an 8 ohm speaker which is the most popular impedance. A 4 ohm speaker will overload it and might damage it because the output current will try to be double. A 16 ohm speaker will work but its max power will be half.
I explained why in my post #61.
Don't you know what "most common impedance" means? It means the impedance that is mostly used. I also explained what happens if you use a different speaker impedance.
A power amplifier is a voltage source. When it has a load impedance that is lower than its recommended impedance then the current is higher which might damage the amplifier. If the load impedance is higher than its recommended impedance then its power into the load is reduced.
I don't make speakers so I don't know why 8 ohms is the most common impedance. Maybe it is easy to make and uses a reasonable amount of voltage and current.
A 4 ohm speaker uses more current and a 16 ohm speaker uses more voltage to produce the same loudness as an 8 ohm speaker.