Listen to Sousa introduce his band playing The Stars and Stripes Forever. Or listen to his band play without him, as he was wary of recordings. Or listen to a take by a more recent symphonic band. Or renditions on the guitar (one, two), the organ (one, two), or the piano (one, two). Or performed by the muppets.
The Historic American Sheet Music archive at the Duke University Library has over 3000 pieces published in the United States available online, from the 1850s up to 1920. Composers represented include well-known names such as Scott Joplin, Irving Berlin, and John Philip Sousa. All the music is now in the public domain, and may be printed and performed freely. [Note: Language or stereotypes may occasionally be NSFW.]