For those interested in learning Morse Code by sound, https://lcwo.net (Learn CW Online) is a good resource, although not really usable on a mobile device.

For building speed, I had the most success listening to random characters via https://morsecode.world/international/trainer/character.html during my morning walks. Another option to consider are the Morse Code Ninja https://morsecode.ninja/ YouTube and Podcast content.

Lastly, if you learn better in more social environments, consider the Long Island CW Club, which, despite its name, has members globally. https://longislandcwclub.org/