The WebM container can carry different audio
WebM commonly carries video and audio, but the extension alone does not identify the exact audio codec or track layout. A clip may include commentary or another language. Inspect the source properties or listen to representative passages before deciding which audio should become MP3.
The MP3 is an audio-only deliverable. It does not include the moving image, subtitles, or chapter points from the video container. If those elements provide context, keep the WebM or provide a transcript and link to the original clip.