One Internet resource not that well known among horn players is the Horn Opera Project website of Ericka Tyner Grodrian. Dr. Grodrian is horn professor at Valparaiso State University in Indiana, and she has turned elements of her Doctoral project into a very useful website (Beyond the Short Call) focused on opera excerpts for horn that have been requested on recent horn auditions for opera orchestras in the United States.
The website is here: http://hornoperaproject.org/
Want to learn more, including notes on which of these excerpts are the most frequently requested on opera auditions? I had the opportunity to interview Dr. Grodrian for episode 20 of the Horn Notes Video Podcast. The episode is below; it took far longer to edit this special episode than any other prior episode for reasons that will be clear as you listen to it, but it did come out well with some great info for horn players. This and other of these video podcasts on a variety of horn topics may also be accessed here.