The play symbol is undoubtedly one of the oldest interface icons. The play icon indicates the direction of movement and is thereby a variation of the arrow symbol. When used in tape recorders the direction of the play arrow would indicate the actual physical direction the tape would move. The play icon has not always had its present form. for instance in the ReVox reel-to-reel recorders in the 1950's the play button featured an arrow with a shaft and a note symbol above it, emphasizing that pushing the button would play the recording. You can find out more about the ReVox and other early play back icons here: http://www.pushclicktouch.com/blog/?p=57.
Reel-to-reel tape recorders 1960's