Voice activation seems not to work

[ophiochos]

I just got a U3 via a US friend. One of the features I wanted was the voice-activation. I have clicked on the setting that says Voice Activation but it does not seem to make any difference - recording does not pause when the room is completely silent. The manual says there is a level to set for this but I only have a checkbox (firmware 1.23 on the device but the manual seems to be version 1).

how can I get this to work please?