One of the places I searched for music to play from our porch for trick-or-treaters was the Internet Archive site at archive.org. The site provides permanent storage for collections of digitized materials, including websites, music, moving images, and nearly three million public-domain books, all of which are freely available to the public. If you ever attempt to follow a link on a webpage only to find it is no longer working, you may be able to find an archived version of the page through the Internet Archive's Wayback Machine; simply put the URL into the search field for the Wayback Machine, which will show you all of the times the page was archived in the past.
You can search the Internet Archive site for "Halloween" and get lots of results, if you are looking for free public domain music that you could use for Halloween. I found Extreme Volume Pop Halloween, which is 46 minutes and 7 seconds of instrumental music described by its creator Tom Baker as:
An album of eclectic , haunting music, inspired by the greatest holiday in the history of the world. Dark ambient passages, beautiful keyboard compositions, and occult music to give you many nights of uneasy dreams are what this free recording are all about! For fans of dark music, gothic orchestrations, horror movie soundtracks.
The titles for the songs it includes are as follows:
- The Inner Sanctum
- Lurking Fear
- Halloween Overture 2008
- Mind of a Killer
- Gypsy Caravan
- The Stalker
- Witchcraft Through the Ages
- Midnight Shadows
- Vlad the Impaler
- Galactic Carnivore
- Here There Be Ghosts
You can download a zip file, Halloween_928, which contains one MP3 file of all the songs or you can listen to the music online.
At his website, Tom Baker describes himself thusly:
an experimental musician dabbling in forms as diverse as metal, punk, noise, ambient, drone, industrial, and aleatoric. He has been featured on a number of different internet radio programs, including The Black Cat Lounge, Stirring the Cauldron with Marla Brooks, and Seps Paranormal Radio. He has written a number of novels, and covered nonfiction topics as diverse as true crime, UFOs, urban legends, ghosts and hauntings, and other paranormal events.
A list of books he has written is available here. Scary Urban Legends (illustrated by John C. Eng), Haunted Indianapolis and Other Indiana Ghost Stories (with Jonathan Titchenal), and Indiana Ghost Folklore might also be interesting Halloween reading.