There is, to my knowledge, only one song on osu! I can actually sing halfway decent. But I might do a second one depending on how drunk I feel when asked.
EDIT: oops sorry I forgot about Alestorm that stuff is golden too.
Double Edit: Also, one person left for the final week makes no sense. Your options are: 4 people left for the final week, and the bottom 3 are dropped, thus declaring the winner, or 3 people left, and the process is the same; whoever gets the most votes wins
If you want this to have any sort of balance you should pick a small handful of (hopefully unbiased within reason) people to judge, and these people should not be told who they are judging (ie they get the sound clip, they get the track info, but not who is submitting it). A popularity contest and a judged competition are at odds. Unlike NATIONALITY Idol the audience here is so limited that it takes little more than a submission by someone with a lot of contacts to go forward
on the other hand if this is all just a clever(ish) guise to coax people out of their shells to bust out them pipes, then bottoms up!