For some software examples with voice using analog FM see http://e2e.ti.com/support/low_power_rf/f/155/t/165695.aspx
We have never done any sensitivity tests using analog FM. The sensitivity in this mode will depend on how you implement the demodulator on the MCU side.
For 4.8kbps using the FIFO you should be able to get a sensitivity slightly better than -120dBm.