Hi Kat
I have just replaced the batteries in our little mouse and she is happily singing once more as clear as ever.

I had to carefully unpick the stitches on her back.
Take out the voice box also unpicking the stitches
in the little bag that encased the box.

Using a small cross-head screwdriver I opened it up
and removed the 3 small button batteries
having identified the type/no we then re-ordered from "7 day shop"
at less than £1(BP)per pack of 10.

Now she is singing again - until we get tired and shut her up!
but I shot a video of her singing in case the voice box ever breaks.
It lasted about 2 years last time with children in the house that's not bad and now we have 7 more batteries enough for 2 more replacements! regards
Jen (Hertford England)