Is stereo a proper noun or pronoun?

"Stereo" is neither a proper noun nor a pronoun. It's a common noun.

Here's why:

* Proper nouns refer to specific people, places, or things (e.g., John, Paris, Amazon). "Stereo" is a general term for a sound reproduction system.

* Pronouns replace nouns (e.g., he, she, it, they). "Stereo" doesn't replace any noun.

Common nouns are general words that refer to a category of things, people, or places. "Stereo" fits this definition because it refers to a type of audio system.