gbamp3

gbamp3 is a full-featured music player for the Game Boy Advance, with MP3 and GAC support.

MP3 files can be put on the GBA without conversion. However, as you might know, the hardware of the GBA is quite limited...
I tell you this because MP3 playback in stereo is totally possible, but the quality is way better when mono is selected.
With MP3 files, mono is actually quite enjoyable when listening to music, while stereo might still allow you to listen to a podcast but is not the preferred choice for listening to music.
Pressing Select lets you change the settings. Here you can switch between stereo and mono playback.

GAC files (Game Boy Advance Cache files) are their own file format. They are derived from MP3 files. The output audio quality of GAC files is way superior to MP3 files.
Stereo playback with high-quality output (32,768 kHz / 44,151 kHz) is completely possible here.
You can convert files from MP3 to GAC with the conversion software for the PC attached here.
I did add the functionality to convert MP3 files to GAC on the GBA itself, but I don't recommend it because it takes a very long time. Do yourself a favor and do the conversion on a PC, but in case you are stuck in the middle of nowhere and want to listen to your favorite album, which you only have available as MP3 on your GBA, go for it.
The user interface of gbamp3 is highly customizable, and I didn't add the conversion settings in the application by default. To enable them, you can access the secret settings by going to the last page of the "About" section in Settings. When you press A there, you can enable conversion on the device.

Why a new file format? (GAC)

What makes MP3 playback so difficult for the GBA is that MP3 files are highly compressed music files. For modern hardware, decompressing MP3 files is no problem, but the GBA is a handheld that is more than 25 years old and was never designed for this in the first place.
GAC files are basically MP3 files that have been decompressed to a certain extent, so that the GBA has less work to do. The drawback is that they are somewhat bigger than MP3 files (about 20–50%, depending on whether you choose 32,768 kHz or 44,151 kHz).

Features

Song Search, Song List, Folder Browser, Albums, Artists, Genres... and different playback modes!
gbamp3 can read all the metadata of your song collection. It even shows the album covers!
After adding new files, go to Settings (Select) > Update Library.

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gbamp3 partially uses the input system originally developed for gbawriter, my free writing and diary tool for the Game Boy Advance.

This input system offers two main advantages:

1. Faster typing: Enter text more efficiently on the GBA.
2. Browse while typing: See and browse matching entries directly on screen as you type. This is especially useful when looking up songs in song search.
   

Flashcart compability:
SuperCard
SuperCard Clones
SuperChis Prime
EZ Flash Omega
EZ Flash Omega DE
Everdrive Flashcarts

Test gbawriter first to see if gbamp3 is compatible:

https://halimj.itch.io/