“Yes, yes, we will have a Pianoforte, as good a one as can be got for 30 Guineas,” wrote the author Jane Austen to her sister Cassandra in 1808, noting that she would practice playing country dances as amusement for their young relatives, “when we have the pleasure of their company.” More than two centuries later, in a Seattle far from anything Austen could ever have imagined, we can now hear those very dances at Benaroya Hall on Friday . “ The Music of Jane Austen ,” a special one-night program at Seattle Symphony in honor of the recent 250 th anniversary of Austen’s birth, will include “Jane Austen’s Songbook,” selections from sheet music transcribed long ago by Austen herself, an enthusiastic amateur musician who diligently practiced on her pianoforte every morning. Scholars have, in recent years, been able to digitize the Austen family’s music books , bringing a taste of Regency-era parlor music to the modern world.…