The music isn't gonna be "good" anyway.
It's similar to the menu music on the Wii.
You need to wait and try a game or something.
If your speakers are actually crackling when you hear game music etc, then you have a hardware issue.
If you're hearing popping and crackling, that's one thing, but you can't expect or judge quality by the sound in the main menu.
If you have access to WiFi, you should download Zelda four swords from the dsi shop for free.
That'll give you something to better judge your sound by.
Good luck. Hope everything works out for you.
I know the pain of opening a new gadget and it not working properly.