basic DIY function generator and scope
Your PC's audio output is really neither bipolar nor DC-biased – it's pretty certainly AC coupled, i.e. there's a capacitor in series with it! (In effect, a headphone port would be centered around ground potential, due to the headphone being connected to ground on its other end.)

That means you can just AC couple it yourself (I recommend doing that to avoid any stray DC current, although the AC output coupling technically renders that redundant) to a biasing of any desired DC voltage, e.g.:







simulate this circuit &ndash; Schematic created using CircuitLab

Although I called it optional, you pretty much always want to have an output voltage buffer in a signal generation setting, so add this block to the output of whatever you build – I've omitted it below for clarity's sake. 
Good thing: even local dealers will stock multi-unit opamps with at least two opamps per IC, so this isn't even any extra component effort.

none of these component values are really critical: A soundcard can drive much larger loads than a couple kΩ; choose the resistor or capacitor larger and get a lower lowest frequency you can pass through.

In a simple case, you could just use a voltage divider to bias your signal:







simulate this circuit

If you need adjustable bias, that can be easily done with a microcontroller like your arduino. Use the PWM unit to generate a specific duty cylce:







simulate this circuit