WHen you get to the point of exporting the sound into a WAV or an MP3, does it involve the machine's sound card in the rendering of the sound into file? I would guess that the CPU handles the export process alone and it's possible to export a track without having a sound card on the machine (just like with video editing and image render)...
The sound card is an I/O device, it only processes outgoing audio from or incoming audio to the computer chipset.
A file export happens entirely on the chipset, it's not output. Therefore a file export operates by processing on the CPU and reading/writing from and to memory (or swap disk / virtual memory) and finally saves to the hard disk.
Some software may prohibit using export functions though, if there's no audio card (at all), because the software may consider itself non-functional in that case. E.g. Pro Tools might do this.
on dsp-accelerated software (pro tools hdx) it may involve the dsp chips, so that you get bit-accurate representation of what you were originally listening to. most of the time an audio interface is simply a way to transform the (already computed) data into audible sound.