For Salzburg, MUC is very convenient. It's about two hours by (cheap) regional train, a bit faster and more expensive on the long distance trains but the regional line is supposed to be a prettier ride. Try to avoid going near there (expensive full hotels, crowds, traffic jams) near the music festival unless you want to participate in the festival and can get tickets. Salzberg itself has a small airport, but I suspect that MUC would be more available and more convenient and probably also so compared to VIE. The train between Munich and Vienna is also good, IIRC about four hours but I haven't done it recently. You could do the train in one direction (maybe upon arrival, when you're jetlagged) and rent a car (one way could be expensive) at the end of your trip when you're not in cities. You don't want a car in Vienna or Salzburg.