Music written before about 500 AD is usually considered “ancient”. Music from ancient Greece and Rome are typical examples of what you might call ancient music. Ancient music is usually based on different scales and tuning systems to those used in music of today. Although relatively little is known about ancient music it almost certainly was monodic, meaning consisted of single melodic lines, perhaps with instruments doubling the vocal part, or one another in the case of instrumental music, or sustaining a single low note as a “drone.”
One of the few remaining examples of ancient music is Epitaph of Seikilos, which was found inscribed on a tombstone in Turkey and is dated around the 1st century AD.
Interestingly, during the Baroque and Renaissance periods, the term “ancient” could be applied to music as recent as 50 years old.