Royalty-free music. Pretty much anything classical and by "the greats" tends to be free to use by somebody. Performances can be copyrighted, but the piece itself cannot. Some choose to protect their rights, others choose to release or "copyleft" it.
I would agree that instrumental music is important. The one exception is if you're trying to indelibly record the era. For example, high school slideshows with rock/whatever music of the time. Not that such productions are necessarily legal, of course -- most use music that is not authorized for use/distribution.
I still have my old videos from high school and college -- and no, I'm not saying who's playing.
Strings can be nice, piano puts me to sleep, jazz/blues is good (no sax solos, however -- zzzzzzzz.)
There's really no rule to style. Just opinion and taste.
One good source for outright free music is the U.S. Library of Congress archives at archive.org.
Plenty to choose from:
http://www.archive.org/browse.php?fi...on=audio_music
I really enjoy the old movies and TV shows archived there (under
Moving Images), for free, legitimately. Forget Netflix and the "blockbuster" garbage made today by Hollywood, churned out to make a buck instead of tell a good story.
Or all the really good radio shows:
http://www.archive.org/details/oldtimeradio
I've also "ripped" music from silent movies. For example, from the
Harold Lloyd box set. (For a personal family slideshow, not anything commercial, as the music may be protected.) Or silents also found at archive.org, which is not protected.
Beyond that, you can search Google for "free royalty free tracks" to find even more places to grab great clips.
You'll find sites like this:
http://incompetech.com/m/c/royalty-free/
Isn't this interesting?
http://incompetech.com/m/c/royalty-f...?genre=African -- listen to the first one, Tafi Maradi
I also like Indian (i.e., Native American, First Nations) music. It can sometimes for for something.
More silent scores:
http://incompetech.com/m/c/royalty-f...20Film%20Score
Youtube is a waste of time. There are MUCH better sources.