From http://www.w3schools.com (Copyright Refsnes Data)

SMIL in Parallel

Objects inside the <par> element will be played at the same time (in parallel).

The Parallel Element <par>

The <par> element can have a number of attributes. The most common attributes are:

Attribute	Value	Description
begin	time	Sets the delay before the element is displayed
dur	time	Sets the duration for the display
endsync	"first"\|"last"\|id(clip)	Synchronizes the stopping of elements
repeatCount	number	Sets the number of repetitions for the display

For a full list of SMIL elements and attributes: W3Schools SMIL Reference.

Synchronization

A parallel group of clips can be stopped at the same time using the endsync attribute in the <par> tag.

endsync="first" stops all the clips in the <par> group when the shortest clip are finished regardles of any time parameters set for the other clips.

endsync="last" concludes the <par> group when all clips have finished playing. This is the default.

endsync="id(ID)" concludes the <par> group when the clip with the identified (ID) clip are finished. The ID is referring to the value of the clips id attribute.

Example: Displaying Things Simultaneously

<html xmlns:t="urn:schemas-microsoft-com:time">
<head>
  <?import namespace="t" implementation="#default#time2">
  <style>.t {behavior: url(#default#time2)}</style>
</head>
<body>

<par>

<t:audio
src="liar.wav"
repeatCount="indefinite"
type="wav" />

<t:seq repeatCount="indefinite">
  <h2 class="t" dur="1s">
  I will display for one second</h2>
  <h2 class="t" dur="2s">
  I will display for two seconds</h2>
</t:seq>

</par>

</body>
</html>

Click here to try it yourself.