16 years & 6 months later, This issue just ocurred to me, It is most likely due a Hardware problem, And in my case the audio wasn't simply playing because the upper screen had to be tilted a bit. To be honest, I bought this NDS Lite 3 years ago, And I don't know how damaged it was, It still worked pretty well nonetheless, Aside from the obvious, The upper screen's hinge is a bit damaged and blunt moves worsen it.
Incidentally, I found a video that explains how to fix this very same error, With a 3DS that is, And it envolves totally on Hardware. If you want me to send the video, DM me, Since the page won't allow me to submitt any links.
As a side note, I think NDS still has some value and 3DS comes very handy even in these days.