# EPUBCheck MED-005: non-standard overlay audio, how to fix it

Source: https://epubcc.com/epubcheck/med-005/ (updated 2026-11-04)

EPUBCheck MED-005 is an error message: “Media Overlay audio reference … to non-standard audio type … found.” A media overlay points to an audio file that is not a core audio type. Narration in media overlays must use MP3, AAC in an MP4 container, or Opus in an Ogg container; EPUBCheck names the file and its type.

- Official message (EPUBCheck 5.4.0): Media Overlay audio reference %1$s to non-standard audio type %2$s found.
- Default severity: Error

## Typical causes

- Narration recorded or exported as WAV, FLAC or another format.
- An audio file with a wrong media type in the manifest.

## How to fix it

1. Convert the narration to MP3 (audio/mpeg) or AAC in MP4 (audio/mp4).
2. Update the src of each audio element in the SMIL file and the manifest item with the new file and media type.
3. If the format is right and only the manifest media type is wrong, correct the media type.
4. Rebuild the EPUB and run EPUBCheck again to confirm the message is gone.

## When it's not this

If the audio URL has a #fragment, the error is MED_014.

## Sources

- EPUB 3.3, §9.2.2.8 The audio element: https://www.w3.org/TR/epub-33/#sec-smil-audio-elem (read 2026-11-04)
- EPUB 3.3, §3.2 Core media types: https://www.w3.org/TR/epub-33/#sec-core-media-types (read 2026-11-04)

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EpubCC.