I have solved the music room but I don't understand how I was supposed to guess that.
What I thought it would be (rot13):
gur abgr ng cbfvgvba a vf yrsg/zvq/evtug vs gur ahzore a oruvaq gur cvax wryyl svfu vf va gur yrsg/zvq/evtug cngpu
What it actually was (rot13):
gur ortvaavat bs "gjvaxyr gjvaxyr yvggyr fgne". jurer jnf gur uvag sbe gung? gurer vf ab oyvaxvat fgne naljurer V fnj.