What does the speaker mean by "I am the audience that witnesses history"?

It means the person is a formal witness, spectator, bystander, etc.

Explanation:

For example:

She witnessed our wedding. to bear witness to; testify to; give or afford evidence of. to attest by one's signature: He witnessed her will.

Answer: "I am the audience that witnesses history" = "I am the person/and or people who have seen much be made that would cause great impact on society"

Explanation:

Though this may not be fully correct, it is my interpretation of the quote "I am the audience that witnesses history"

To be an audience means you are the people or person watching something, a movie or drama.

And to witness history, I see it as meaning you have seen something or watched something go down or happen that would effect a lot of people and be known for much after much is gone.