1. Daisy Bell (Bicycle Built for Two) - Possibly the best example of this trope as it is the most obvious. The song was first composed by Harry Dacre in 1892 and performed by Dan W. Quinn in 1893, but that isn't why you know this song. You know this song because it was sung by an IBM 7090 in 1961 making it the "first song sung by a computer". Since then, it has become associated with robots, and, more specifically, sentient robots such as BonziBuddy singing it when prompted, Hal 9000 singing it during deactivation, or Bender singing it in Futurama. 

2. Toreador March - Originally written for a scene in the play Carmen, you don't know the Toreador March for its use there but instead in the FNaF series and has gone on to be associated more with mascot/friendly horror than its original play.

3. Auld Lang Syne - What started as a Scottish folk song became the anthem of New Year's Eve when played by Guy Lombardo at the stroke of midnight on the 31st of December every year since 1929, the 1947 version of the song still being played in NYC to this day. 

4. Steamboat Bill - Originally a simple American folk song about a boat race became associated with Mickey Mouse and the greater Disney company when used in the short Steamboat Willie, itself a loose parody of the film Steamboat Bill, Jr.

Music being given a new cultural connotation due to media