Q: Macbook Air running loud and slow, volume on mute.

My Macbook Air has only just recently started running quite slow and once the the computer is turned on, the fan becomes really loud even though there's not a lot going on. The volume also doesn't work and when you go onto system preferences it shows that the mute box is ticked. When you try to turn the volume up it just shows a symbol like 'ø', even when headphones are plugged in. Hoping you can help because I need to use the sound for school. Thanks