If you take the # 20, 21, or 22 tram from Sandstraße to Karlstor Nord and then the #19 tram to National Theatre, it's a 1 zone (Innenraum) single trip ticket for 2,70€. As Marco indicates there is a time limit to complete your journey.
Or you can walk ½ km east on Gabelsbergerstr. to Luisenstr., and take bus 100 to Odeonsplatz, then walk another ½ km to the National Theatre. That's a Kurzstrecke (short trip, up to 4 stops) ticket on bus 100 for 1,40€.
Or, for 6,20€, you can purchase an Innenraum Tageskarte (all day ticket for the inner zone) and make unlimited trips in the inner zone, including round trip from Sandstr. to the National Theatre and back.
You can look up schedules and fares on the Munich Metro website, www.mvv-muenchen.de.