Second example of SSML media element.

media-example2.xml

<speak>
  <par>
    <media xml:id='question'>
      <speak>
        When was Wikipedia launched? <break time='5s'/>
        Was it 2000?<break time='2s'/>
        Or 2001 <break time='2s'/>
        Or 2002? <break time='2s'/>
        What is your answer?<break time='5s'/>
        <emphasis level="moderate">It was 2001!</emphasis>
        Wikipedia was launched on 15 January 2001, two days after the domain was registered by Jimmy Wales and Larry Sanger.
      </speak>
    </media>
    <media repeatDur='60s' soundLevel="-15dB" end='question.end+0s'>
      <audio src="https://storage.googleapis.com/aogssml/background_music_loop.ogg"/>
    </media>
  </par>
</speak>

this feature is already requested, please vonte it on https://alexa.uservoice.com/forums/906892-alexa-skills-developer-voice-and-vote/suggestions/34035127-the-possibility-to-add-background-music-with-the-h