Overall music history
http://www.musicmap.info/ This website is pretty cool. Be sure to use the tools on the left to navigate to get more details.
https://en.wikipedia.org/wiki/Music#History
Film music
https://en.wikipedia.org/wiki/Film_score#History
http://www.americancomposers.org/hollywood_chihara_article.htm
http://www.classicfm.com/discover/film-music/
Classical
https://en.wikipedia.org/wiki/Classical_music#History
http://www.naxos.com/education/brief_history.asp
http://www.newworldencyclopedia.org/entry/Classical_music
Opera
https://en.wikipedia.org/wiki/Opera#History
http://www.opera-europa.org/en/opera-resources/history
http://www.greensboroopera.org/oft-history.shtml
March
https://en.wikipedia.org/wiki/March_(music)#History
https://www.britannica.com/art/march-music
http://wiki.kidzsearch.com/wiki/March_(music)
Music theatre
https://en.wikipedia.org/wiki/Musical_theatre#History
https://www.musicals101.com/stagecap.htm
http://www.theatrehistory.com/american/musical030.html
http://www.newworldencyclopedia.org/entry/Musical_Theater
Pop
https://en.wikipedia.org/wiki/Pop_music
http://www.newworldencyclopedia.org/entry/Pop_music
http://www.shinemusic.com.au/musicresources/history-of-pop-music.aspx
http://www.brighthubeducation.com/social-studies-help/123260-studying-pop-culture-music-evolution/
Rock
http://rock.about.com/od/rockmusic101/a/RockHistory.htm
http://www.streetdirectory.com/travel_guide/31436/music/history_of_rock_music.html
https://en.wikipedia.org/wiki/Rock_music
Folk
https://en.wikipedia.org/wiki/Folk_music
http://www.music-folk.com/american-folk-music/
https://www.britannica.com/art/folk-music
Spiritual/gospel
http://www.newworldencyclopedia.org/entry/Gospel_music
http://gospelmusicheritage.org/site/history/
http://www.earlygospel.com/eg-origins.htm
Country
https://en.wikipedia.org/wiki/Country_music