A good way to protect your speaker from overloading is by using a limiter circuit. This circuit protects your speaker by limiting the input signal to the amplifier by a certain level.
This circuit uses an IC from New Japan Radio Inc. It is NJM2761 which is specially designed for limiting audio signals. It is designed for powered speaker system to protect the speaker from overloading.
The circuit is very simple. It only requires 7 capacitors and 1 resistor for setting the limiter level.
Rsense resistor sets the output limit voltage. Rsense value is anywhere between 4k to 20k. Rsense can be computed as:
Rsense = 4000/Vlimit
Vlimit is the limiting output voltage. Output voltage limit can be from 200mV to 1V. Use 4k if you want to have 1V limit.
Operating voltage is 2.7V to 13V so you need to have a regulator if you are using an unregulated 12V supply because an unregulated 12V power supply level can vary up to about 15V. You can also use the popular 7805 voltage regulator to power this circuit.
DataSheet link: NJM2761 Data Sheet
IC information link: NJM2761 Product Page