Fix errant music filename which crashed the sound on Linux.

When Minecraft tries to play a music file and doesn't find it, it throws the following exception:

    Exception in thread "Thread-3" java.lang.NullPointerException
       at paulscode.sound.codecs.CodecJOrbis.initialize(CodecJOrbis.java:259)
       at paulscode.sound.libraries.SourceLWJGLOpenAL.play(SourceLWJGLOpenAL.java:616)
    (...)

... and the sound thread is dead until the next restart. On Window this doesn't happen because the filesystem in case-insensitive, unlike on Linux.