\definerenderingwindow
  [example]
  [width=0pt,height=0pt,frame=off]

\setupinteraction[state=start]

\useexternalrendering
  [mysound]
  [audio/mpeg] % audio/x-mp3
  [akkerman.mp3]
  []

\definereference [StartSoundTrack] [StartRendering{mysound}]
\definereference [StopSoundTrack]  [StopRendering{mysound}]

\definelayer[resources][width=\paperwidth,height=\paperheight]

\setupbackgrounds[page][background=resources]

\starttext

    \setlayer[resources]{\placerenderingwindow[example][mysound]} % once

    \startTEXpage
        \button{start}[StartSoundTrack]
        \button{stop}[StopSoundTrack]
    \stopTEXpage

    \startTEXpage
        \button{start}[StartSoundTrack]
        \button{stop}[StopSoundTrack]
    \stopTEXpage

\stoptext
