Castle Crashers uses the Windows default audio device to output audio; I'd wager that your audio output device isn't set to default.
To check this (on Windows 7):
1. Enter Control Panel from the Start Menu
2. Then, select: Hardware & Sound > Sound > Manage Audio Devices
Your default device will have a green check. If there's no green check on the device you're using, right click the name of the device and set it to default.
If this doesn't fix your issue, you should also ensure that none of your peripherals (specifically Webcams) are interfering in some way.
- Shut down Steam completely - In your Windows tray on the bottom right of your screen, find the Steam icon, right click on it and select Exit.
- Unplug any webcams you have connected.
- Launch Steam.
- Launch Castle Crashers.