Some motherboard with onboard sound card have jumper setting on the motherboard near the sound ports, make sure you did not accidentally remove or change the setting while moving the motherboard to the new case. Check the speaker plug and power; make sure they are correctly plugged in. Check the BIOS to make sure the onboard sound is enable. Check device manager if there are any error, reinstall the driver if you have to. Check your volume control.
The old case, did you forget anything? You said it had speaker port in the front, maybe it might be part of your sound card that you might need?