If you have the DS volume turned down, sometimes it distorts the sound, especially on DS games. Try the volume at a higher level and see if it's still there.
I did notice it in the GBA game Metal Slug at various volumes for the explosions. I was just wondering if they are hitting a range sensitive to the DS.
Didn't notice it on the GBASP..... but then again... the sound is much clearer and crisper on the DS.
I'll have to try Asphalt GT at a higher volume again and see. AT lower volumes it did it.