Impressions

I've had the occasion to hear several Romance languages mumbled carelessly and hastily on their home turf: Quebec French, Romanian, Spanish and Romansh. To my ear, and out of intelligible earshot, each one sounded pretty much like the others. This blend struck my ear as a sort of retrograde to the early Romance that had just spun off from Vulgar Latin. There must be a de facto law of atrophy at work here.

Language geeks (but not linguists, as far as I know) sometimes say there's some sort law of stable complexity about language: what it loses in complexity somewhere, it gains somewhere else.

So, while The modern romance languages have lost part of the grammatical complexity of latin, they have greatly gained in the way of phonological (at least fro some of them) and syntactical complexity.

That richness I have heard in abundance when a language is spoken carefully and even with pride-- by museum docents in Schuls, Switzerland (Romansh) and a museum village in rural Romania. An elderly vendor at the foot of Bran Castle spoke less formal but wonderfully clear Romanian as he gave me a shot of homemade plumb brandy to toast his homeland. Even the hurried merchants of Bucharest spoke distinctly -- if rapidly.

It is in the mouths of the apathetic or otherwise unconcerned (or trendy teenager) people of the population that the "mushy" language seems to thrive.
No surprise there, but that is where (to my outsider's ear) the Romance languages blend into a generic mumble.

Just curious: How different does this mumbled language sound from mumbled non-Romance languages?

The Romance mumblings I've heard all have the cadence of that late-Latin, early Romance syllabic stress. In other words, you can perceive the source through a glass darkly. I have heard some mumbled Vietnamese: some of my Vietnamese friends who came to the US after the fall of Saigon in 1975 pointed out that some the "other guys" in their group weren't clear speakers of their language. Again, as I listened, the Vietnamese tonal pattern was there, if muddled -- and it was nothing like muddled Romance or slurred English.

There's probably a lot of subjectivity there, but that's the nature of impressions, I suppose ...