Hey, in Tarsdorf, there have been Sommerauers since 1600! Unfortunately they are listed online only till 1879!
All Sommerauers that were born later, you have to look up in the "Landesarchiv" in Linz (capital of Upperaustria). For information, i think you could write them an email...
If you have more information on your mothers parents or grandparents, i'm sure we could find their birth-/death-/marriage-information online here: http://www.matricula.findbuch.net/php/main.php?ar_id=3670
my familytree now contains over 400 members and i nearly all found them on the website above! Have a try!