Turn on transistor with output from sound card
The problem with your circuit is that to turn the transistor on, its base voltage needs to overcome both the B-E drop of the transistor and the voltage drop across the LED.  To get to that level, the audio input needs to be one more diode drop higher due to the diode before the base.  That comes out to around 3.5 V, which is more than the normal high points "line level" audio, except at some sound peaks.

Here is a possible method that requires audio amplitude that should be within the normal range:



The peaks of the audio signal only need to be 700 mV or so to turn on Q1.  When that happens, C1 is discharged and the bottom end of R1 held low.  That turns on Q2, which pulls up OUT.  R3 only illustrates a load.  R2 is to overcome any leakage of Q1 and to make sure Q2 is solidly off when there is no audio signal.  It can be omitted in a cost-optimized system.

When audio is no longer present, C1 will charge up thru R1 with a time constant of 10 ms.  That allows for plenty of time for peaks of new audio cycles to discharge C1 again before Q2 turns off.  The response from audio going away to OUT going low should be a few 10s of ms.