Search: daft punk; one more time
Why: They wear robot helmets when they perform live, which is rarely. I can imagine them recording robot-y voices for, like, "Harder Faster Better Stronger," but I can't picture a robot singing the dulcet tones in "One More Time."
Answer: Sometimes they do, and sometimes other people do! The vocals on "One More Time" are done by Romanthony, an American DJ. According the liner notes for "Harder Faster Better Stronger," it's a "robot vocal performance" by Daft Punk and a sample of "Cola Bottle Baby" by Edwin Birdsong.
Of the other songs that aren't all instrumental, I would guess they are probably the same - some them, some others. On "Around the World":
The key hook is a steady bassline and a vocoder-processed voice singing "around the world" in continuous chains. The phrase occurs 144 times in the album version and 80 times in the radio edit.Source: Wikipedia
The More You Know: The 2 robots in Daft Punk (oh PS, did you see them in new TRON?) are 36 year-old Frenchmen Thomas Bangalter and Guy-Manuel de Homem-Christo, who has the same birthday as me. Here is what they look like under those things:
