yea there is some songs and the only way you can get them is by playing co op or use a cheat code to get them.

if you have two controllers (like one normal and one guitar) then just hve someone play the controller on easy and u play the guitar untill u get the songs u want.

the cheat code is the fastest way and all it will do is unlock those songs

YO, RB, RO, GB, RY, YO, RY, RB, GY, GY, YB, YB, YO, YO, YB, Y, R, RY, R, Y, O

thats the code for all songs