the Bayern-Ticket is not valid beyond the German/Austrian border at Mittenwald, i.e. you would need a separate onward ticket here. Salzburg itself, however, can be reached with a Bayern-Ticket from Munich, it is rather popular to do a daytrip this way. the Bayern-Ticket currently costs 25 Euro (+2 Euro if you buy it at a counter instead of online or vending machines). an important detail is that as a measure against fraud, users must put their name on the ticket.
Would the route be possible…

