The Validator is pointing to the following line in the 'Content document'(html file):
<h1 id="bajw_0004"><a href="bajw0004.smil#bajw_0004"></a></h1>
This line is for a heading that is no longer being used - it has no text typed into it unlike the following example:
<h1 id="bajw_0002"><a href="bajw0002.smil#bajw_0002">Heading One</a></h1>
The line with the error is pointing to a SMIL link that no longer exists and is no longer a part of this particular DAISY project.