You unlock all the hidden content by beating the game on different difficulties. Most are unlocked by beating the game on normal, however there are a few that require you to beat the game on expert.
CFA 44 Nosferatu Fighter - Beat the game on Hard difficulty. It will appear in your hangar, purchase it for $229,500.
Music Player - Beat the game once.
Scene Viewer - Beat the game once.
Unlock Expert Difficulty- Beat the game once.
Unlock Ace Difficulty - Beat the game on expert.
SP New Game - Complete campaign once on any difficulty setting.
you also need to unlock all planes and colors.