IPA: /ˈsaʊndɪŋli/
KK: /ˈsaʊndɪŋli/
In a way that produces a loud noise or sound.
The thunder sounded soundingly, shaking the windows.
Soundingly → It is formed from "sound" (from Old English *sund*, meaning healthy or whole) and "-ly" (meaning in a manner of). The word "soundingly" means in a manner that is healthy or thorough.
Think of 'sound' as being healthy or whole, and remember that 'soundingly' means doing something in a thorough or healthy manner.