Generally it’s cheapest to buy individual tickets as far ahead as possible rather than a pass.
You do not need a pass, which by the way is often more expensive than buying discounted tickets, or local savers.
No need to buy in advance, simply purchase it from a ticket vending machine in Munich. Just do a research beforehand and familiarize yourself with the few constraints (e.g. 9am rule on weekdays, train categories permitted).
From Salzburg to Zell am See as well as from Zell am See to Innsbruck you can make use of the "Einfach Raus Ticket' of Austrian Railways ÖBB, which is similar to the Bavaria Ticket. Again, take some time and read about the constraints (again 9am rule on weekdays, train categories permitted).Edited: 10:12 am, June 10, 2018
Ruling out eurorail pass
Can someone explain the difference between Bavarian and einfach raus tickets.
Those tickets are a lot cheaper, so what's the deal? Is it valid only on slow trains.
Please re-read what has been explained in reply #2, and do a search on these products using Google as well as the search function of the forum. Info can also be found on the offical websites:
www.bahn.de (German Railways)
www.oebb.at/en (Austrian Railways)
Einfach-Raus-Ticket = good for train travel in Austria (all states).Edited: 1:00 pm, June 10, 2018
Bavaria ticket only valid after 9am on weekdays. No restrictions weekend. Also covers transport within Munich. Good for groups up to 5. Only valid on regional trains which are only a few minutes slower than Railjet services.
Einfachraus good for groups but restricted to the booked train only.
As Michael says—-all info is online