bfo vs Non-bfo
Some low freq beacons are modulated in such a way that an additonal oscillator to produce an audio frequency tone.
The Glossary of "Aircraft Radio Systems", J. Powell says that a Beat Frequency is the difference frequency resulting when two sinusoids are mixed in a non linear device, ie modulator.
Some very basic low freq beacons, mostly in the third world, are unmodulated and the bfo switch on the receiver applies a reference radio frequency to the incomming signal and you hear the diference, or audio portion.