I think the level is more rig dependent than anything else. It depends on whether you apply the signal to the rig's mike connector or to any auxiliary connector on the back.

Sound cards typically have 1 volt peak output on line out jack which can be controlled by the Microsoft audio control panel or similar for other operating systems. If you are using a commercially made interface box or a homemade one, they usually have a level control on the front panel as well. If you are going straight from the sound card to the rig, you may need a resistor attenuator in line.