MacBook Air sound disappearing

The sound on my MacBook Air keeps randomly dropping off completely. Sometimes when I power up my sound is working, other times there is nothing at all. Mute isn't on, I can still turn the audio up & down but it has no actual affect on the sound and at no point have I changed the audio settings.

It's relatively new (mid November) and I've already had to have it repaired 2-3 weeks after I bought it as the cursor kept disappearing, the battery life already seems dire too.

Am I doing something wrong re.the sound and can I fix it?