Matt, the Eurail Select Pass covers your transportation but not your accommodations. You probably pay a little for a seat on the train; you definitely pay for a sleeping accommodation. This is a "supplement", sometimes erroneously referred to as a "reservation" because with the supplement your accommodations are reserved.
Supplements are by the night, not by the distance. Generally, the supplement for a couchette in a 6 person compartment will run about $40; in a 4 person compartment around $50. A sleeper will run $100 or more. These are only rough numbers and are if purchased in Europe or by phone or online from one of the European national railroads. Supplements from a stateside seller, like RailEurope, will be appreciably higher.
As for Munich to Vienna, day trains make that trip in as little as 4+ hours, so a night train is hardly necessary. Additionally, you will probably need to take a local train to catch a night train (I couldn't find one direct from Munich to Vienna), and your actual time on the night train would hardly be enough for a good night's sleep.
