An insider on the IGN forums has said the following:
“The system works using an advanced voice recognition system which Nintendo has been developing for several years. When a user speaks the sound is converted into on screen text and sent to the other players. However, rather than just plain text being sent the system also picks up the tone, pitch and volume of the voice and translates that into colour, font size and character set. For example, if a player shouts ‘I win’ loudly into the microphone the on screen text may be all capitalized, and in a large red font; conversely if a player speaks quietly the font may be a pastel shade and smaller.”

According to Krazygamers.com, this is the same member that said there was going to be a speaker on the Wiimote before it was announced by Nintendo.

And with this Patent the rumor could be true.