Every few months the Rhode Island Philharmonic welcomes music aficionados in for open rehearsals, late afternoon events on the evening before a big performance. For $15, listeners this week can watch guest conductor Tania Miller and the orchestra finalize their program of Mozart, Beethoven, Mendelssohn, and two different Strausses. It’s a waltz-heavy affair that includes On the Beautiful Blue Danube, surely the most widely recognized piece of the evening.