As you know, Voice123 is a marketplace that works primarily as a 'connection service' between Talents and Buyers. The casting process is totally free to use, the only cost involved is what you as a Buyer, pay to the Talent. When the casting process is over and a talent has been selected or hired, the negotiation about payment, deadlines and NDA is between Talent and Buyer. Voice123 is NOT part of the final transaction.
After the auditioning process, Voice123's system can be used as a communication tool with the Talents, but all payment details are worked out between the Buyers and Talents directly. It is up to the Talent and the Buyer to negotiate the actual rate, possible buyouts, and residuals.
If you have selected the talent you want to work with, send them a message letting them know they are hired. Also set deadlines, file formats, agreements and payment options.