Are you flying into Munich on June 13, or will you already be there. If you are flying in, buy the Bayern-Ticket at the airport. It will be valid, after 9 AM, for you travel by S-Bahn to Munich Ostbahnhof (or the Hbf) and for a regional train to Salzburg.
If you buy a Regio120 Ticket in Wittenberg, it will be valid all day (Sunday) for any regional train. There are REs leaving at 8:12 and 10:12 and later. There are no reserved seats on regional trains, but the REs originate in Wittenberg, so if you get there early (20 min?) they will arrive empty at the platform, so you should be able to find seats.
If you want a reserved seat, take a long distance train. There is an ICE leaving at 8:52. Fare for each adult (over 14) is 19 euro; children under 15 travel free but must be declared when you book.
You didn't say the size (number and ages) of your "family". The Regio 120 Ticket might not be the appropriate ticket, depending on number and ages in your party. You can enter the travelers with ages on the Bahn website and it will tell you the appropriate ticket.