As someone who is only fluent in English but tried to study French for a couple weeks before visting Paris many years ago... Spoken French has institutionalized mumbling; what I read is you're supposed to form the final consonants of words with your mouth, but not voice them... Which ends up in practice with the consonants being pretty much voiced if the next word starts with a vowel and not if not... But I'm not surprised there's some variance.

The best written reference I could find (which sadly, I can't find again) was from a Hong Kong university where the writers seemed to have learned both English and French analytically rather than natively, so they would explain the rule for French and compare with the rule(s) for English and expose me to the rules I unconciously followed without knowing they existed.

Studying written and spoken French simultaneously was pretty helpful for me, otherwise it was too hard to fill in the ending letters of many words as needed.