Here's why:
* Common nouns refer to general things or people, not specific ones.
* Proper nouns are specific names of people, places, or things.
Since "speeches" refers to a general category of oral presentations, it's a common noun.
However, if you were referring to a specific collection of speeches, like "The Gettysburg Address and other speeches by Abraham Lincoln," then "speeches" would be part of a proper noun phrase.