There are always one or more SMIL files present in the DTB fileset.
The SMIL files contain information about sequences of media objects (text elements in XML documents, segments of audio files, images) to present to the user in a synchronized (parallel) manner.
In the SMIL file, the element used to define a timing sequence is the <seq>. The element used to define parallel (temporally synchronized) timing is the <par>.
[...]
<seq>
<par id="par01">
<text src="document.html#fragment1" id="text01"/>
<audio src="audio1.mp3" id="audio01"/>
</par>
<par id="par02">
<text src="document.html#fragment2" id="text02"/>
<audio src="audio2.mp3" id="audio02"/>
</par>
</seq>
[...]
Using the SMIL files only, a DAISY playback device can make a linear sequential presentation of the whole book, that is, play it from beginning to end. The SMIL files contain all the sequence and syncronization information needed.