Any song in the game can easily be "sung" to 100% by blowing steadily into the microphone on Easy difficulty. A fan should work for this, but if it doesn't seem to be working, simply exhale (inhaling works too) into the microphone during the singing parts.
Finn (the quarterback, the late Corey Monteith) is featured in three of the songs you must complete to 95% or better: all three songs should be available for selection from the song menu by default.