PC sound generation of step waveform
This sounds like there is some DC Bias removal taking place. In general with audio you want your average voltage should be 0v. This is because of how speakers work. 0v would mean that the speaker is at rest. In order for the speaker to sound normal you want it to be able to go back to its resting position.

In order to achieve this, sound cards will remove any DC offset in order to set the average voltage to be 0. This can be done both digital and analog. If it is done digitally there may be some settings with your driver to disable DC Bias adjustment. If it is done analog, you will have a tough time getting rid of it.