Does changing the volume slider do anything for this? A lot of DS games had poor audio mastering as well but if it does it in menu then that is probably not at play here.
Also when you say distorted what do you mean? Clipping (going beyond max volume), noise, unclear audio where it is clear elsewhere*
I am not aware of any common "oh it is the capcitor on ?" type failure methods like I might have for various other pieces of audio gear, however I can not rule out something basic like some of those happening here either. Equally the audio systems are so underpowered on these systems that the vast majority of the damage we ever see is liquid damage or drop damage.
*classic one being slightly broken headphone port where sound comes through but is unclear
