What is the typical max voltage out of a PC speaker Jack?
I just measured the voltage from my smartphone headphone jack, which should be similar to what you'd get from a PC headphone jack.  I got +/- 1.5v (3.0 volts peak to peak).  This was not under controlled conditions with a known audio source.  I would guess that the output could peak at higher than +/- 3v (6v peak to peak).