Modern audiences are used to performances that try to get as close as possible to a composer’s intentions. So it will likely be a shock when Leonard Slatkin performs Beethoven in versions as interpreted by Mahler. M”ahler, like many before and after him, simply filled in places where notes were missing. The range of most of the woodwind instruments had increased, too, so Mahler used the added notes to keep the flutes, say, from having to drop an octave for a note or two.”