From the manual:
FREQOUT works best with a 20MHz or 40MHz oscillator. It can also work with a
10MHz or 8MHz oscillator and even at 4MHz, although it will start to get very hard
to filter and be of fairly low amplitude. Any other frequency will cause FREQOUT to
generate a frequency that is a ratio of the actual oscillator used and 20MHz.
For simple beep type of tone from a piezo buzzer just use PWM, HPWM or "manually" run the (E)CCP module.

/Henrik.