All credit goes to https://github.com/umd-mith/webvtt-player. This is a slightly modified version to allow choosing audio files through a dropdown menu or uploading them from a local device.
You can create a pull request to upload the files you want (beware of copyright
restrictions though). Just place the .mp3 files and the .vtt transcripts in
the data/ directory by following the existing directory structure. Then, run
ls data > playlist.txt or manually add the directory name in playlist.txt.
Be sure to also include a reference to the audio source. After the pull request
is reviewed and merged, you will be able to select your audio file at
https://yongfu.name/shadow.