One of the problems with performing music from earlier eras and cultures is that the notation always leaves many details out and it is the oral (aural?) tradition that explains how to fill in the missing information.

Still, the music fares here much better than, say, ballet. Many ballets were created, especially in the XIXth century, but almost all of them are lost hopelessly, because adequate dance notation was never invented. What remains is a handful of classical ballets that continue to exist through continuous rehearsal, led by a few theaters.