server.R

# Javascript functions to set audio source, play and pause the audio
js_set_audio_src = "shinyjs.setAudioSrc = function(source){document.getElementById('audio_player').src=source};"
js_pause_audio = "shinyjs.pauseAudio = function(){document.getElementById('audio_player').pause()};"
js_play_audio = "shinyjs.playAudio = function(){document.getElementById('audio_player').play()};"

# Values stores the last point so we can turn off the audio
Values<-reactiveValues(old="Start")

# Preview the track when the user clicks on the graph point
# Pause the track if it's  the same point
observe({

  if(length(input$tooltip)==0){
    return()
  }
  
  if(input$tooltip != ""){
    full_name = unlist(strsplit(input$tooltip,split = "<br>"))[1]
    if(isolate({input$tooltip == Values$old})){
      js$playAudio()
    }else{
      preview_url = tracks[tracks$full_name==full_name, 'preview_url']
      js$setAudioSrc(preview_url)
      isolate({ Values$old<-input$tooltip })
    }
  }else{
    js$pauseAudio()
  }
})