Fn-F1 - 'turn sound on/off'. The speaker icon at the bottom right of the main desktop will either be a speaker with lines coming out of it (sound on), or a speaker with an “X” over it (sound off).
Fn-F2 - 'LCD contrast'. Holding Fn while pressing F2 will toggle through four levels of screen contrast. A bar graph will display on the bottom portion of the screen indicating the level.
Fn-F3 - 'turn vibration on/off'. Each press will generate either a short vibration (off), or a long vibration (on). There does not appear to be a screen indicator of this status.
Fn-F4 - 'alarm on/off'. This will emit an alarm “beep” (alarm is on) and the alarm “bell” icon will appear. Pressing again will generate the same sound, but the alarm icon will go away (alarm is off).
Fn-F5 - 'radio mode on/off.' This appears to enable or disable the transmitter. When the transmitter is off, an icon of two Cybikos with an “X” between their antenna will appear in the lower left of the screen. When the transmitter is on, that icon is replaced by a face with the number of Cybikos in range.
Fn-F6 - 'enable/disable Power Saver mode'
Fn-F7 - 'mail check.' At least with the latest software (1.3.55), pressing Fn-F7 will display (briefly) a message along the bottom of the screen showing your mail status such as “unread: 0, outbox: 0”.