# What is EPUB media overlays?

Source: https://epublab.com/glossary/media-overlays/ (updated 2026-11-03)

Media overlays synchronise recorded narration with the text of an EPUB 3, using SMIL files that pair audio clips with elements of the content documents. Reading apps highlight each phrase as it is read aloud.

## Why it matters when you convert a PDF to EPUB

A PDF carries no narration, so converting one never produces media overlays; they are added later, for read-along books. They explain the MED messages in EPUBCheck, which check that clips and text references match.