I know that C. S. Lewis describes Reepicheep's voice as shrill and piping, but it never works for me when I try to imagine it that way or in the Radio Theater version. Not when it comes to his dramatic lines anyway and he has a lot of those. I wouldn't want it to be too deep either because that might be comical. But in all honesty, I'd rather it be really deep than really high. 

In the Star Wars universe, Yoda has a high voice, in the first movie he appeared in at least. I imagine Reepicheep the same way (but with a courtly English accent, not faux-Chinese)

I always do Reepicheep's voice as fairly high-pitched when reading the stories out loud — higher than my normal speaking voice (as an adult woman with a relatively deep voice), but not to the point of making him sound like Alvin and the Chipmunks (I'm dating myself there ). For me it needs to sound like a voice that could credibly come from a mouse — one who's much larger than ordinary non-talking mice, but still a mouse — and yet not sound too absurd, although his characterisation in the books is really quite comical, just not in an outright silly way. In both the books Reepicheep appears in, a big theme is the contrast between his small size and his huge bravery (or outright recklessness), brought out by the fact that he's extremely self-conscious about being so small and extremely concerned about not being taken seriously because of it:

"Highest of all High Kings," said Reepicheep, "permit me to remind you that a very small size has been bestowed on us Mice, and if we did not guard our dignity, some (who weigh worth by inches) would allow themselves very unsuitable pleasantries at our expense. That is why I have been at some pains to make it known that no one who does not wish to feel this sword as near his heart as I can reach shall talk in my presence about Traps or Toasted Cheese or Candles: no, Sir — not the tallest fool in Narnia!" Here he glared very fiercely up at Wimbleweather, but the Giant, who was always a stage behind everyone else, had not yet discovered what was being talked about down at his feet, and so missed the point. (PC, ch. 15, pp. 177-8 in the Puffin edition)

There's a lot of comedy in that speech and many of Reepicheep's other utterances — affectionate comedy, because we can't help admiring him and loving him to bits (I can't, anyway), but we're still laughing — and for me, a good deal of that effect is carried by it being delivered in the kind of voice you'd expect such a small creature to have, in sheer contrast with his bravado. That doesn't come across nearly as well if his words are delivered in a baritone register! But as I said, it does also need to be done in a way that isn't too silly. Not an easy task and it'll be interesting to see what the next adaptations of PC and VDT do with him.

Posted by: @courtenay

I always do Reepicheep's voice as fairly high-pitched when reading the stories out loud — higher than my normal speaking voice (as an adult woman with a relatively deep voice), but not to the point of making him sound like Alvin and the Chipmunks (I'm dating myself there ).

I'm guessing you talk in your "head voice" when you do Reepicheep. It's most common to talk to in chest voice. I think a lot of us tend to sing in the voice that we speak in ("chest voice"). Singing in "head voice" takes a lot of practice and training.

Anyhow, I could see how they would use Reepicheep talking in a higher voice. I think in the Focus Family Radio Theatre, Robert Benfield, who voiced the Adult Edmund at the end of LWW and HHB, also voiced Reepicheep, and he spoke in a higher voice. I think he used his head voice to talk for Reepicheep.

Posted by: @courtenay

I always do Reepicheep's voice as fairly high-pitched when reading the stories out loud — higher than my normal speaking voice (as an adult woman with a relatively deep voice), but not to the point of making him sound like Alvin and the Chipmunks (I'm dating myself there ).

Aww I wish I could hear you do it, you probably do a really good job.

@nicemice2023 Well, I dunno about that... I can't guarantee he doesn't come out with an Aussie accent!!  (I still have one, even after over 10 years of living in England.)