Power LED from headphone jack
Vin through R1 through D1 through R2 to base Q1
 C1 from D1 - R2 junction to ground
 +5V through R3 through LED1 to Q1C  

Q1 any NPN higher beta better. Emitter to ground.
 R1 about 100R  - just stops cct loading Vin much.
 C1 say 19 uF - 100 uF.
 D1 = 1N4148 or anything.
 R2 = 10k
 R3 = 220 ohms.
  Q emitter to ground.

Whoever may feel free to draw up circuit.
 Vin R1 charges C1 via R1 &amp; D1.
 Vootage on C1 turns on transistor via R2. 

Vin_peak ~~~= 4 x sqrt(power) for 8 ohm system.



Try this - just maybe.
 Here the LED is powered BY the audio sugnal directly.

Vin through R1 through D1 through LED to ground.
 C1 from D1.LED join to ground.  

D1 = 1N4148 or anything.
 C1 say 10 uFto 100 uF.
 R1 SOT( select on test) say 470 ohms to start.