Poor audio in one speaker - Mac
s g,

One thing in your troubleshooting leads me to believe that this is not a sound card issue: headphones worked.  The sound card controls all audio, not just the speakers.  

I see two possibilities(off the top of my head):


Your left speaker is damaged.
Your headphone jack has something in it that's causing electrical problems, or has a damaged connector.  Make sure there's no debris jammed in there or other damage.


Also, electrical feedback from the bad speaker (if that's the case) could cause the behavior you saw with the right speaker.  Again, I don't see this being an sound card issue at all.

Honestly, I would take it in and let Apple have a look at it.  If it is hardware issue, they can repair it.  If it's something in the software, they may have a solution.