What Am I Riddles

Question: My eye is dark,
and my mane is yellow.
You man see me at the mart,
and I'm probably taller than you fellow.

What am I?

Question: My shallow hills are the faces of kings.
My horizon is always near.
My music sends men to the grave.
My absence sends men to work.

What am I?

Question: I am a nice word composed of five letters, but the Romans would call me a numbers.

What am I?

Question: It takes one word to separate them; otherwise they are inseparable.

What are they?

Question: Whoever makes me can hear me, but nobody else can.

What am I?