Clock Pulse from Audio Signal (Laptop Headphone Jack)
If all you want is a TTL clock signal, this is easy. The best choice would be to be a comparator IC. The output is typically open collector. Pull it up to 5V. Ground the inverting pin, and run your sound card signal into the non-inverting pin. Just leave the signal as a sine, the output will be square and 50% duty cycle.

My next choice would be to set an op amp up as a comparator, and power it off 5V single supply. You'll want to use a CMOS op amp for this.

Using your transistor would take a little work, but it wouldn't be my first choice of component for this task.