You should follow the format used by the prompts - assuming G711 that would be 8 bit sample size, mono, 8kHz sample rate which is 64 kbps bit rate.
Unity can play just about anything back if it's in a codec supported on the box it's installed on, but a huge sample rate and stereo (which I've seen studios do for folks in the field) really sounds horrible. Don't do that.
Also, make sure they target about -26 dB - it's best to send them a prompt WAV file to base it against. Biggest problem I see with 3rd party studios is the volume being way off and blowing people's ears off or having to be converted down which reduces sound quality.
