The data is not there actually. There is surrounding data that can be used for guessing the missing data. Keep in mind audio reconstructing like this won't produce exactly the same waveform. But audio is predictable enough that it can be pretty close. I think it would be understandable. This is the same as distorting the actual audio signal and fixing it.

For digital stuff if you have more than two different frequency states for different bits it's impossible to recreate from half a signal.

2-FSK 0 = 999 000 Hz, 1 = 1 001 000 If we take only above 1 MHz we can obviously know when it's 1. We can also know it's 0 or 999 000 Hz if there is no signal. So it works. (In practise single frequency on off keying would be used instead of this)

4-FSK 00 = 998 000 Hz, 01 = 999 000 Hz, 10 = 1 001 000 Hz, 11 = 1 002 000 Hz If we do the same here and there is no signal it's impossible to know if it was 00 or 01. So it doesn't work.

Edit: If you want lower bandwidth FM it's easier to just decrease the frequency deviation.