There's two issues in fact, related to the thumbnails:

1) For non-image assets, the word thumbnail obstructs asset name:

The word comes from the IMG alt attribute:

2) The source for the thumbnail links to a non-existing HTML page, instead of an img file

a) (for audio): src="/magnoliaAuthor/dam/travel-demo/audio.html"

b) (for video) src="/magnoliaAuthor/dam/travel-demo/video.html"

2019-12-12 08:43:06,813 WARN  info.magnolia.dam.core.download.DamDownloadServlet: Could not retrieve an asset based on the following pathInfo /travel-demo/audio.html
java.lang.IllegalArgumentException: Node '/travel-demo/audio' is not defining an asset but another item type
	at info.magnolia.dam.jcr.JcrAssetProvider.createAsset(JcrAssetProvider.java:167) ~[magnolia-dam-jcr-3.0-SNAPSHOT.jar:?]
	at info.magnolia.dam.jcr.JcrAssetProvider.getAsset(JcrAssetProvider.java:156) ~[magnolia-dam-jcr-3.0-SNAPSHOT.jar:?]

For sure the src cannot be an HTML page. The HTML SPECS say:

(https://www.w3.org/TR/html52/semantics-embedded-content.html#the-img-element)

Solution proposal (should fix both issues described)

If we cannot generate asset thumbnails for these types of assets from the assets themselves (for sure not for the audios) we should use our own icons instead, from our Icons font:

https://documentation.magnolia-cms.com/display/DOCS61/Icons#Icons-Imageicons