On https://freeaudioconverter.net, you can:

• Convert an audio file to another format - MP3, AAC, WAV, Opus, Vorbis (in the .mka container), FLAC, ALAC, AC3, DTS or CAF.
• Convert a video to an audio-only file (to any of the above formats).
• Convert a video to the MP4 or MKV format.
• Change the audio codec of a video to MP3, AAC, AC3, DTS, WAV, FLAC or ALAC.
• Trim a video or audio file (will not work if using the Safari browser).
• Download a YouTube video or the audio only. The webpage is a youtube-dl wrapper.

• You can see the file upload progress as a percentage and also amount uploaded (MB) in realtime.
• Upload completion time is shown in realtime.
• Whilst the file is being converted, you can see how far into the file the encoder currently is. This information is updated every second.

• Download as an MP3 or MP4 file, or simply the best quality video/audio stream that is available.
• If you choose to download as an MP3, the thumbnail of the video gets embedded as the cover art.
• Download the best quality audio stream without encoding it, so no lossy-to-lossy encoding is done (only applicable if you use the "Audio [best]" button.

Many filetypes are supported, click here for details. Support for other filetypes may be added, feel free to contact me to enquire.

The file trimmer will not work when using the Safari browser as it does not support the HTML <input type="time">. I may try to find another user-friendly way to allow the user to input their desired start and end time that is compatible with all browsers. Feel free to submit a pull request if you know of a good alternative implementation that is compatible with all browsers.

FFmpeg

LAME v3.100 | https://lame.sourceforge.io/

fdk-aac | https://github.com/mstorsjo/fdk-aac

opusenc opus-tools 0.2 (using libopus 1.3.1) | https://github.com/xiph/opus

libvorbis

youtube-dl | https://github.com/ytdl-org/youtube-dl

Video [MP4] -f "bestvideo[ext=mp4]+bestaudio[ext=m4a]/best[ext=mp4]/best"

Video [best quality] youtube-dl <video_id>

Audio [MP3] -x --embed-thumbnail --audio-format mp3 --audio-quality 0

Audio [best] -x <video_id>

  --enable-gpl
  --enable-libfdk-aac 
  --enable-nonfree 
  --enable-libmp3lame 
  --enable-libopus 
  --enable-libvorbis 
  --enable-libvpx 
  --enable-libx264 
  --enable-libx265

Python 3.6+

pip install youtube-dl

pip install -r requirements.txt

Contributors are welcome, simply submit a pull request.