Rockbox is a free replacement firmware for digital audio players (MP3 players). Its enhancements include support for pretty much all codecs, advanced sound settings, applications, utilities and games.
Check if your device has a usable Rockbox port: https://www.rockbox.org/.
The official tool to manage Rockbox can be installed with the package. Rockbox Utility can install the replacement boot loader, the main firmware, and any extra features like fonts, themes and applications.
Installing Rockbox on your device
The Rockbox project has fantastic install instructions. You can find them at https://www.rockbox.org/manual.shtml.
It is recommended to install the boot loader using #Rockbox Utility. For Rockbox Utility to detect your player, it must have write access to the internal disk of the player.
# mount -o uid=1000,gid=1000 /dev/sdb /mnt.
#Rockbox Utility can make backups of your device.