How about a build a bear with the talking heart with the recording Goodnight X that sibling can press & listen to.
When you are deciding what to do, definitely have one eye on the difference between the future grief and bereavement position of a very young child that is a sibling and your own grief and bereavement (or an adult's grief and bereavement from the perspective of an adult with life experience and greater understanding of death).
I know you will be focussed on other things right now but if you have time or in a break, it really might be worth speaking to Winston's Wish or those specialist in children experiencing bereavement for guidance.
What I would be concerned about is that something tangible (like a bear with a recorded voice) might mean more to an adult than it does to a child and become an unhealthy talisman - that the child feels subconsciously 'pressured' to love - and does not help either child or adult.
I don't know because I am not a specialist in child bereavement but do tread carefully.
The advantage of photograph and videos and voice recordings is that they are 'normal' part of life and how we can really capture a person. They are then there to access or not as a growing sibling wishes to.
A bear with a voice recording in it, if the child is not receptive or doesn't like it, will become a problem for everyone because decisions about what to do with it (throw it away (no!), destroy it (no!) put it in the attic (no!)) will become deeply overlayed with grief emotion.
What may be right for some people may be wrong for others. The same applies to casts of hands/feet - some peope will love them as beautiful, some people will find them creepy. Everyone is different and children have that added vulnerability from a lack of deep understanding.