These two questions are totally different. The voice one is easy -- if it has a smaller voice box, it will likely have a higher pitched voice due to physics. However it would be trivial to construct a small creature with a large voice box. The vocal tract is a small portion of human from.
Intelligence is a more difficult question, given that we really aren't very good at defining what intelligence is. However, you may enjoy working with the Sentience Quotient (SQ). It was a logarithmic measure of processing rate vs mass. If you can process more data with less mass, you get a higher score. A few interesting data points:
* Most plants score a -2 (very little processing per unit mass)
* Carniverous plants cluster around +1 (more processing than other plants)
* Humans score a +13, and most animals cluster around that
* A Cray-1 scores a +9
* IBM Watson scores a +11 or +12
The theoretical limits of SQ are -70, processing 1 bit in the lifetime of the universe, using all the mass in the known universe, and +50, which is a limit by quantum mechanics.
Thus, if you want to shrink the size of a human brain by 10 fold, you need a 10 fold increase in processing per unit mass. SQ is on a log scale, so that means you would need a SQ of 14, one higher than humans. Clearly this suggests that one could get quite small with advanced processors like Josephson Junctions (which score a +23), but if you want to stick to using neurons like you or I have in our head, you can't get much smaller without compromising SQ.
Now as to other approaches to increase intelligence, being more efficient with what you have, that's an open ended question. You could play a lot with that, and maybe even explore the difference between intelligence and wisdom. However, SQ is a decent numeric approach you can play with.