I think this is attributed simply to the transfer of the DVD and the home sound system, versus the different theater venues, prints and sound systems.
I don't think the film was altered in any way prior to DVD release, and it sounds exactly the same on my commercial release DVD as on the Academy screener I have had since November.
I saw the film in multiple theaters and two of them had slight alternations in their sound -- Dolby to THX, mixes between speakers that wrapped around versus just in front, audio too low, etc., and on a few occasions the "I swear" line was not audible and companions quickly asked, "What did he say?"
It would seem an improbability that they would loop this line for the DVD in a different way, it's just not a standard thing to do and the explanation is likely a variance in sound systems.
rt