Assuming you're not travelling in July or August (when the bus schedules are reduced), there are two options to get from Rome to Volterra, in each case connecting in Cecina:
OPTION 1: one connection
Rome Termini dp. 10:09 regionale train Cecina ar. 13:27 Cecina dp. 14:57 CPT Pisa Bus Line no. 790 Volterra ar. 15:55
Fare: 15.50 plus 3.50 Euros (train/bus) Days of operation: Monday through Friday
OPTION 2: two connections
Rome Termini dp. 10:09 regionale train Cecina ar. 13:27 Cecina dp. 14:19 regionale train Saline ar. 14:51 Saline dp. 14:55 CPT Pisa Bus Line no. 790 Volterra ar. 15:15
Fare: 16.35 plus 1.45 Euros (2 trains/bus) Days of operation: Monday through Saturday
NOTE: The bus connections are at the Cecina train station or at the Saline train station.
NOTE: Schedules on Sunday are different.
NOTE: In option 2, the train fare is based on ONE ticket from Roma Termini to Saline di Volterra.
BEWARE: The bus schedules are reduced in July and August. You need to consult the CPT Pisa Bus Lines website noted below.
I don't recommend connecting in Florence. You would need to take two buses (two different companies, and with a connection at Colle Val d'Elsa), and the times don't work out. Also, the ES/AV train between Rome and Florence is much more expensive than the regionale train between Rome and Cecina (or Saline).