I also think that FFC used it as a device, just as they used the approaching el train. At the point when Sollozzo is really talking, Michael's thoughts are all rushing around in his head. He's not really paying attention to Sollozzo, but is content to let him babble on, since he's about to kill him. Michael is nervous, and if you listen carefully, the sound makes it seem as if Sollozzo is almost mumbling. I believe the lack of subtitles was a device to show Michael's state of mind at that moment.