What's the amplifier power, and are you "using" the power?
Higher power makes this easier but it means you also have to protect the LED.
Impedance isn't an issue since you always have a current-limiting resistor in series with an LED, and that resistor is much higher than 4 or 8 Ohms.
Here are a couple of circuits you can try -
The regular diode is a rectifier so the capacitor can charge-up to a positive voltage and it prevents the capacitor from discharging back into the amplifier... The capacitor stores energy and the diode insures that ALL of its energy is discharged into the LED.
The 220-Ohm resistor is the usual current-limiting resistor for the LED, and it also limits current into the capacitor. Otherwise the capacitor would "short out" the amplifier.
A larger (more uF) capacitor will hold the LED on longer, and you might need more than 1000uF. But a larger capacitor also takes more time to charge-up.
With the "low power" circuit, LED brightness WILL vary with loudness.
In the "high power" circuit the Zener diode "clamps" the voltage to 5V maximum. This holds a more-constant brightness and it protects the LED. The 100-Ohm resistor protects the amplifier and the zener.