It wasn't the chest cold that gave you the sexy voice, it was the attendant irritation of your vocal chords. The irritation, and possible covering of mucus makes the chords thicker, and just as a bass string on a guitar vibrates at a lower frequency for a particular string tension than a thiner string, so do your vocal chords.

So all you need to do to obtain a lower voice is to irritate your vocal chords so that they swell slightly. My voice, after singing my heart out in the audience at a rock festival, is a good 4-6 semitomes lower the next day!