I had to return BtVS season one because it had two copies of the same disk, and I tried the copy of Phonebooth that I rented on two different DVD players with the same problem - all of the phone call shots with the protagonist talking to someone else had no sound from the other people’s end. It was strange because once in a while a few words of the other actors’ dialogue could be heard, which is the only way I knew they were really speaking aloud the entire time. Not to mention they’re supposed to be sending everyone who bought the 6th season of the X-files a replacement disk for the “triangle” episode which they later noticed transfer errors in… I was supposed to get it in May, but I’m still waiting 
So yeah, quality control could be better, but it’s probably no worse than any other product. Not that it’s much of an excuse.