"Can be", but normally is not. Fully differential circuits do not use ground as a reference; it is only a leakage path.
But this leakage path to a common voltage (sometimes ground) is
need often a good thing to have.
Happens all the time and usually works fine, though without the safety ground/shield/chassis connection you'll lose some common-mode noise rejection.
If input our output is using a transformer its fine. they have near infinite common mode resistance and good common mode suppression.
A non ground referenced devices is Capacitive coupled to some ware between mains and neutral.
worst case it can be 240V and supply up to 0.5mA (depending on local regulations)
more typical would be 120V or 1/2 mains and ~0.1mA
But this would still put a huge common mode voltage on an typical line receiver.
Lets say its a 100k, so 50k in common mode. so common mode voltage can be 5-25V? (EDIT: teste my speakers its it's 200k in my case)
Or 13 to 25dBV assuming the receiver can Handel 25dBV common mode
Whats a typical CMMR for a powered monitor? maybe 30dB?
So in this case -17 to -5dBV noise can be added by not using the shield as a ground reference
If you don't think that's a fair assumption or not typical feel free to bring your own numbers to the table and make your own calculations.
Your normal seems to be different to my normal.
I tested all my equipment in reach and sure pin 1 is always connected to chassis ground by default (some has ground lift function)
Also ti app note calling it "Typical balanced audio-transmission system"