Archive.org and DigitalHistory.UH.edu offer license-free instrumental sheet music for non-commercial use, as of 2015. Unless an artist designates their work as public domain, all recorded music in the United States is under copyright through 2067.
To display a list of license-free items in the music catalog at Archive.org, search for "instrumental public domain recordings." Selections are available to play online, share to social media, and embed into blogs or other websites. Download options include Ogg Vorbis, Torrent and variable bit rate MP3 and MPU. Archive.org provides an extensive selection of public domain entries, mostly in the new artist category.
Browse the University of Houston's Digital History Project website to view music recorded earlier than 1927. Categories include music from the 1920s, Gilded Age, Irish, jazz and Tin Pan Alley. The site cautions that content is for non-commercial educational use only, fitting in with the timelines defined in the United States Copyright Act of 1976. PDInfo.com provides an overview of the Act and licensing issues covering sound recordings.
To find instrumental music for commercial uses, such as the background for a video or film, Musopen.com suggests purchasing a lifetime license. Most licenses classify under Creative Commons 2.0 or 3.0, requiring acknowledging the original artist or other license owner, but providing freedom in their use. A lifetime license authorizes repeated use.