To think my MIL hates me because. . . .

[roweeena]

She bought my 2 yr old DS a karaoke machine - surely you would only inflict this sort of torture on a mortal enemy!

If I hear old macdonald one more time . . .

Take it on every single visit to her house. And then 'forget' it for long enough that it just stays there because your DS loves singing songs for his Grandma...
That'll teach her

She hates you. Earplugs?