DS Lite speakers are fine but startup sound isn't?

Something weird with this DS I just repaired: the speakers seem fine when playing a game and navigating the menu, but the startup sound is high-pitched and just kinda crappy. Adding more to the strangeness, the startup sound is fine when I'm first setting up the DS, but it goes weird after that. What is going on here? (I have 2 DS Lites with the exact same problem)