Hi Jack,
You must have deaf people working at a PC repair bench or it is too noisy there for them to hear if the PC speaker output is working, right? :lol:
Don't use only one LED with a series resistor, or else the LED's max reverse voltage rating of only 5V might be exceeded. Use two LEDs, connected back-to-back with one wired backwards to the other.
The LEDs and their series resistor shouldn't be connected "inline" with a speaker, they must be "tapped" into both wires of a speaker. Therefore an extension cable can be used is if it has one plug to the PC and 2 jacks, one for a speaker and the other for the LEDs and their resistor. Or if you aren't using a speaker then an extension cable can have the LEDs and their series resistor connected to its end instead of a jack. :lol: