The speaker out of the card usually isn't as good quality as the line out. It is usually louder though. There would be some point on the mainboard before the output amplifier where the line out signal would exist. You could tap off the signal at this point if you dont mind some fine soldering. To find this point you could get the data sheet for the sound chip ot the amplifier chip on the mainboard. However I wouldnt go to this much trouble unless you find the sound quality not up to scratch.