The letters in Greek are always pronounced in the same way. So, ρ is always R. Any wrong sounds are due to problems with the audio which we are expecting to be changed soon.
Btw there are some double letters which sound different for example: NT=D but those are a few exceptions.
See here for lots of info on the Greek alphabet:
HOW TO ADD THE GREEK ALPHABET TO YOUR KEYBOARD
HINTS ON HOW TO LEARN THE MODERN GREEK ALPHABET and HOW TO GET THE GREEK KEYBOARD https://forum.duolingo.com/comment/23430853
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM
And check out the Greek Forum here with more links.