The sound seems pretty good; but I still couldn't tell it was the plural versions of "triangle" and "circle". What's the difference in sound I should be looking for?
It's the paltalization of the <n> and the <l>. One reason why it's important to distinguish between broad and slender consonants (a lot of plurals are formed by making the final consonant slender)
...so Pól doesn't like this book, hey. It is full of triangles! :o