Section 7.3 The nav element: restrictions states:

The child a or span element MUST provide a non-zero-length text label after concatenation of all child content and application of white space normalization rules. When determining compliance with this requirement, the concatenated label MUST include text content contained in title or alt attributes for non-textual descendant elements.

If an a or span element contains instances of HTML embedded content [html] that do not provide intrinsic text alternatives, the element MUST also contain a title attribute with an alternate text rendering of the link label.

These rules could use an example to clarify what constitutes a valid text label in the presence of nested child elements and embedded content, like

<nav epub:type="…">
   <h1>…</h1>
   <ol>
      <li>
         <a href="chap1.xhtml">
            <audio src="..."/>
            <audio src="..." title="Chapter 1"/>
         </a>
      </li>
      <li>
         <span>
             <img src="..." alt="Chapter 2"/>
         </span>
         <ol>...</ol>
      </li>
   </ol>
</nav>