The first thing you have to realize about the music of Golden Sun is that when the game was first released, the only way to play the game was on the very first iteration of the Game Boy Advance. This system only had 1 actual speaker and yet the music managed to sound absolutely phenomenal. In these 3 games, Motoi Sakuraba goes on to compose some of the finest music he has ever done. Sadly, to this day, no official soundtrack of either Game Boy Advance game has ever been released. There are plenty of direct game-rips out on The Internet, though.
Golden Sun (The Broken Seal) and Golden Sun: The Lost Age
If you thought the main theme was good, check out this very sad, minor version that plays during the scene where the heroes find Vale destroyed. It just suits the scene perfectly... (The video title is a spoiler, so be careful!)
The second games also gives us Moving Worlds, an upbeat remix of the main theme, which plays during the password input.
If you think the original version of the Elemental Star Chamber theme is CMOA, just wait until you hear this version of it.
There's also a beautiful, soothing remix of "Elemental Stars Chamber" by Chaotic Marin that utilizes the soundfont of Pokémon Ruby and Sapphire, exemplifying the underwater thematic element of Ruby/Sapphire while retaining the mystery and intrigue present in the numerous locales of Weyard and Hoenn. It's sure to send chills up the spine of Golden Sun and Franchise/Pokemon fans alike.
One of the most epic tunes in the first two games is the one that plays in the Venus Lighthouse.
This track. The tense beginning, the timid flute urging for your progress before some unspeakable catastrophe falls upon you or your village... It's short, yet it's incredibly efficient in setting a feeling of urgency and hovering danger.
Prox. A gloomy, grim, cold theme. It fits Prox perfectly.