When querying the API for audio thumbnails (yeah, appears to be stupid ...) it returns a height and width in a manner that let the thumb look distorted.

Example:
https://commons.wikimedia.org/w/api.php?action=query&format=jsonfm&iiprop=url|size|timestamp|user|sha1|comment|mime&iiurlheight=120&iiurlwidth=120&intoken=&prop=imageinfo&titles=File:Ca-Rosa_Lavi%C3%B1a_Carreras-article.flac

Result:
"thumburl": "https://bits.wikimedia.org/static-1.25wmf22/resources/assets/file-type-icons/fileicon-ogg.png",
"thumbwidth": 120,
"thumbheight": 13,

https://bits.wikimedia.org/static-1.25wmf22/resources/assets/file-type-icons/fileicon-ogg.png is a 120x120px file which is then squashed to 13px height