I guess they meant microphone + speaker response. I think speakers could be inefficient to produce low frequency sounds, as every speaker of every cell phone out there. They reverberate and lose sound power. In the other side, it is possible we could use high frequency (+20Khz) instead of low frequency, and then we could also freak some dogs... (there are already "dog whistle" apps for some other devices). It's clear these little speakers work better at high frequencies.
For secret communications we can use SMS... or we can make an app (that works both on Symbian and Maemo, ie, QT4) that generates and reads this kind of sounds and translates 'em as text... It would be useful inside a room, not farther...