audio: Re-convert ding.mp3 from original source.

Similar to zulip.mp3 in the previous commit, this file is corrupted
and only playable in some browsers.

Re-download it from the source[^1], and convert it to MP3 via
`ffmpeg`.

[^1]: https://freesound.org/people/InspectorJ/sounds/411089/ per docs/THIRDPARTY