Detecting state of LED to control another circuit
I agree with Eugene Sh - cover the LED with an opaque tube and put a phototransistor inside. You'll want to ensure that it is tuned to the visible light spectrum as opposed to IR, which many are (for IR remote control applications). Something like the Vishay TEPT5600 would be ideal. Here is a starting point circuit for you:







simulate this circuit &ndash; Schematic created using CircuitLab

Q1 is the phototransistor - when enough light strikes it it will start to pull the gate of M1 up to 5V, turning it on. M1 is chosen to have a relatively low threshold voltage (2.1V) so it should be easy enough to turn on hard, switching on D1 (your new indicator). R1 allows you to set the sensitivity.

Since I don't know anything about the Klaxon you are hoping to drive, I've included a relay with a clamping diode D2 which can be used to switch on a separate circuit with its own power supply. You'll need to choose a relay with a 5V coil with a coil current lower than about 400mA, which can switch whatever voltage / current you need for the Klaxon.

If you know what you're doing, you could possibly drive both the LED and Klaxon with M1 simultaneously, but I'll leave that up to you.