Weird one that!
I'd remove the palmrest and check the left hand speaker hasn't got a screw / piece of metal debris "stuck" to the cone through magnetic attraction of the speaker.
To remove the palmrest you need to remove the keyboard first. For a How to on this download the How To Doc list from my sig and from that download the T4x Screws doc.
Also get a mini headphone jack and plug it in and out the blue (green?) left hand jack hole a few times; sometimes dirt falls into the internal contacts and can do funny things.
Also while you're at it if you have headphones check what the LH headphone channel's doing.
If it's OK and there's no metal debris on the cone, it may be is a distorted LH speaker cone and you may need to change the speaker assembly.
If you want to try a replacement speaker assembly give me a PM with your name and address details and I'll pop one in the post (I've got enough to go twice round the world....!!) - no charge.
If all that fails, download and burn a live Mint CD
, load a live (non installation) Linux Mint desktop and then test the sound through that.