AudioLemon found this video which may be the first use of an electronic instrument in a film soundtrack – L’Idee (1932).
It features prominent use of the Ondes Martenot, an early electronic instrument, in a score by Arthur Honegger.
The First Electronic Music Soundtrack – L’Idea (1932)
[dailymotion]http://www.dailymotion.com/video/xevud_lidee-1932_shortfilms[/dailymotion]
