As far as I recall the Opcode Studio series are MIDI interfaces, with no audio connections at all.
For recording spoken audio, you should be OK with a decent Microphone direct into your computer mic inputs. Very few mics are stereo, and one person talking is pretty much a mono source anyway. Mic inputs do analogue to digital conversion- not always very prettily, but they do it.
The next step up would be to go for a large diaphragm capacitor mic, which would need an interface or a small mixer to supply 48 volts phantom power, or you could be happier with a USB mic.
Having said all that, I would always include music and sound effects as well, so the story is just beginning.
A few references:-
Wikipedia on Opcode
An audio interface
USB Mic

Cheers
Dave