If you want to change the voices then you've got alot of work ahead mate
In the DATA\Audio folder there is a series of UTF files containing the generic space pilot radio chatter. The all begin with "pilot_something".
I suggest using the XML project to extract them all out.
From there you need to create new entries into the "voices" ini files and create new trees in the UTF files with matching names (this can be done in the XML file created).
Careful though....after extracting the files there's thousands of files in them....
But for just single stations it should be alright.
FL usually supports any WAV container file (can be uncompressed PCM or compressed as mp3). I'd say that they're 32kbps, 22khz, mono Mpeg-Layer3.
I was going to create all new voices for Freeworlds but considering the number required I decided not to.