Hi PammyBoy,
While the circuit kinjalgp supplied is good, there is one major oversight. On all computers nowadays, the sound card requires powered speakers to produce descent sound, therefore a tiny current is output from the card (in the region of 20mA @ 1V I believe).
Being an Electronic forum, the solution above (to use the Accessibility Features to flash the screen) is too simple... ;-)
To operate correctly, this circuit needs a power source to drive the LED, in this case 2x1.5v AA batteries and a transistor and other minor components. The transistor should be a high gain unit, such as my favourite BC108 (they are still making them???), and should be provided an input at it's Base from the PC's sound card.
Hope you don't mind the hand-drawing.
If you find that the sensitivity of the circuit is more or less than what is required, drop in a variable resistor instead of R2. I built and tested this only 10 mins ago, and it worked OK off a small radio I have. Best of luck...
Eoin.