Plot hole: When Mr. Jeffries finds out that Evan is named August Rush too, he goes to the Central Park concert and sees Evan onstage, however he couldn't know that Evan was having a concert, or even that Evan was at that place at that time.
Suggested correction: There were streetlight banners all over Central Park with August Rush's name on them (and presumably the Philharmonic had flyers and other ads with August's name on them). So it's certainly conceivable that Jeffries recognized the name from an advertisement.
Plot hole: When the chipmunks finally release their first song, there is a montage showing the chipmunks rising musical success. In one shot we see a marquee promoting "The Chipmunks Premiere CD Release", and on it is a picture of the chipmunks in the clothes Dave has made them (note Alvin in his 'A' jumper.) However, Dave doesn't actually make the chipmunks these clothes until further on in the film. (00:41:50)