Typically a speaker will have an impedance (resistance) rating printed on the back of it; typically 8 ohms but sometimes 2, 4, 16, or 32 ohms or even some other value (you can also find out using a multimeter). You can solder a resistor in series with the speaker to reduce its volume.
For example, if you have an 8 ohm speaker and you solder an 8 ohm resistor in series then the speaker will be supplied with 1/4 the amount of power that it did previously.
The audio signal is a certain voltage, and the equation for power is (V^2)/R. Since the total resistance connected to the amplifier is now 16 ohms, the amplifier will be providing half as much power as it did when only the 8 ohm speaker was connected. However, that power is now being divided equally between the speaker and the resistor, so the speaker puts out 1/4 the amount of power compared to without the resistor. The speaker converts electrical power into sound, and the resistor converts electrical power into heat.
Speaking of heat, when sizing a resistor make sure to take into account the power rating of it. For example if you have an 8 ohm 4 watt speaker and decide to use an 8 ohm resistor, make sure the resistor is rated for a minimum of 1 watt. If the speaker is rated for 1 watt, then using a 0.25 watt resistor is suitable (we have all kinds of resistors at VHS, as well as soldering equipment available to use. Lee’s Electronics also sells all of these supplies.)