Parallel Playlist
The SMIL parallel playlist is a list of media objects that start playback simultaneously.
Children of the parallel playlist can be specified to start at a specific "wallclock" time defined by the player's real-time clock. See the section on Wallclock for details.
Basic parallel playback
The following example will play both <seq> playlists at the same time. Each <seq> is playing in its respective region.
<par>
    <seq repeatCount="indefinite">
        <img src="pic1.jpg" dur="5s" region="main" />
        <img src="pic2.jpg" dur="5s" region="main" />
        <img src="pic3.jpg" dur="5s" region="main" />
    </seq>
    <seq repeatCount="indefinite">
        <img src="side1.jpg" dur="5s" region="side" />
        <img src="side2.jpg" dur="5s" region="side" />
        <img src="side3.jpg" dur="5s" region="side" />
    </seq>
</par>
Slide Show with Background music
Audio tag is not yet supported.
<par>
  <seq repeatCount="indefinite">
    <img src="pic1.jpg" dur="5s" />
    <img src="pic2.jpg" dur="5s" />
    <img src="pic3.jpg" dur="5s" />
  </seq>
  <audio src="music.mp3" repeatCount="indefinite" />
</par>
The parallel schedule has two children: a sequential playlist containing 3 still images, and a single audio media object. The sequential playlist and the audio object start simultaneously, achieving the effect of a slide show of 3 photos while music plays in the background. The sequential playlist is the simplest form of playlists in SMIL.
In a sequential playlist, media objects are played in the order they are listed in the SMIL playlist. One media object starts playing after the proceeding one ends.
Source: a-smil.org