How do you speak up when you have laryngitis?

Carefully. You need to power your voice from your diaphragm in order to get your swollen cords to vibrate. You should try to speak in a much lower pitch as this will also help the cords to vibrate. The thing you don't want to do is strain your voice or tighten your neck/throat muscles. This can lead to damage of your cords. Ideally, you want to rest your voice when you have laryngitis.
It takes come effort. Acute laryngitis can be due to an acute vocal strain (yelling, screaming) bacterial or viral upper respiratory infection. Voice will be raspy, fatigues easily and mildly painful. It usually recovers with tincture of time and humidification. Sometimes steroids are used once patients are examined. If laryngitis persists beyond 4 weeks, you should see your local ENT professional.