Bus/train from Rome to Volterra on a Sunday?

You can go to Pontedera via train. First you would take a fast Freccia train to Florence (90 min from Rome). Then in Florence you would switch to a train going to Pisa. Make sure it's a train that stops at Pontedera. Pontedera is on the Florence-Pisa train line, but some trains make no stops between Florence and Pisa.
From Pontedera there is a bus (one only on Sundays), which departs Pontedera at 19:30 and arrives at Volterra at 20:51.
Alternatively you could go to Colle Val D'Elsa and take the only bus from there departing Colle at 16:45 and arriving at Volterra at 17:35.
To get to Colle VE from Rome the fastest way is still to get to Florence first. Then from Florence you take a bus to Colle. On Sundays there is more than one bus from Florence to Colle. The absolute latest for your needs departs Florence (next to the train station) at 15:40 and gets to Colle at 16:45, exactly when the Volterra bus departs. The bus to Volterra will probably wait for the Florence bus to arrive before departing, but if you don't want to take chances, you can take the 12:40 bus from Florence which gets you to Colle at 13:45.
If everything goes wrong and you miss your only bus from Colle to Volterra, don't despair. Colle is only 28 km (18 miles) from Volterra.
If worse comes to worst you can call Adriano Guerranti-Taxi service in Colle at this mobile phone number:
+39335 5718753.
He'll take you to Volterra probably for under 50 euro.

I do not see any difference between the DB and Trenitalia schedules. Both show a train from Termini to Cecina with a bus connection to Volterra.

The bus from Cecina to Volterra does not operate on Sundays.

Thanks so much for the many helpful details, Roberto. I am assuming that the bus tickets can be purchased at the local stations the day of, correct? With the Freccia tickets, is there an advantage to purchasing those in advance?
Thanks again.

Yes. Bus tickets can be purchased on board. Ask the driver.
Freccia tickets are on sale online 120 days prior to departure.
Buying online gives the ability to purchase at a discount (ECONOMY and SUPERECONOMY)
Discounted tickets tend to sell out quickly.
However SUPERECONOMY tickets are non refundable and non changeable. If you miss the train you had reservation for, you will lose your money. ECONOMY tickets can be changed for a different train up to the day of departure, but you will have to pay for the change and pay full fare.
If you don't want to take the risk and want to pay full fare, you can buy even the same day at the station. There are so many trains going from Rome to Florence that they never run out of seats. At most they run out of the STANDARD class (the cheapest class of tickets), in which case you can buy a higher class fare (Business or premium) or simply wait for the next train that has STANDARD seats still available (generally 30 min. later). If you buy at the station the day before, you'll certainly find a STANDARD fare for any train you want.
The standard fare Rome-Florence is €45 o/w.
SuperEconomy is as low as €9

Thank you, thank you! SO helpful!
Ken