Variables in credits

[Slanderous]

You can use classical music without worries. But bear in mind to avoid using copyrighted stuff, it might bring problems to potential let's players, like Rom said. You can either use vanilla game's music (you are also allowed to use music from Penumbra's and Machine for Pigs), use royalty free music or ask someone to compose some tunes for you. But yeah, you can use classical music in any way you want to.