Waterproof MP3 player with Bone Conduction Headphones for Swimming - H2O Audio
Normalize MP3 files to play at the same volume - batch processing - YouTube
Amazon.com: MP3 Player with Bluetooth | Portable Music Player with Headphones | Micro SD Card Expandable | Colour Screen, Easy Controls, Lock Button, Sports Clip | Majority 16GB Bluetooth Media Player : Electronics
Understanding the MP3 Format
How to Normalize or Change the Volume of MP3 Files in Windows 10
Splitting a recording into separate tracks - Audacity Manual
Set the same volume to multiple mp3 at once files with MP3Gain - YouTube
Batch Convert Many Audio Files to Equal Volume Levels - YouTube