To me, it sounds like a flapped T or D that often happens with T or D occurring intevocalically in an unstressed syllable in English, and I agree that in many other languages (including Latin, I assume), the flapped "T/D" is a variant of R, not of T or D. Also, I think this male speaker in the audio often pronounces the R in iter as if it's the English R in air. I doubt that I should model my pronunciation of Classical Latin on his.
I was under the impression early on that ablative was used with most prepositions, but as I started learning a lot of prepositions take accusative. That's just the way the language works. Accusative seems to be used more often with prepositions of motion (to, into, across) and ablative more "stationary" (in, with, for). That's not meant as a rule, just a pattern that has helped me.