If your MacBook is working with headphones (from a separate output of mini-DIN/similar type I am guessing) that means your sound card is probably doing fine. You may have a problem for USB output only. There is one other simple potential explanation...
One common mistake is that people sometimes have their L/R output balance set wrong....check you Audio Settings in MacOS, you may find, like the WindowsXP/Vista equivalent, that there is a balance L/R slider that is set to output sound only to your Left speaker channel. On Windows this slider can be moved inadvertently with a mouse-drag or keyboard shortcut. You may want to check this before anything else
In the event the above is not the problem, you may have a bad right channel output when using USB only in which case I'd suggest taking the laptop to a certified Apple repair technician and having them test it...
Write me an email if you want to talk about this further...
One common mistake is that people sometimes have their L/R output balance set wrong....check you Audio Settings in MacOS, you may find, like the WindowsXP/Vista equivalent, that there is a balance L/R slider that is set to output sound only to your Left speaker channel. On Windows this slider can be moved inadvertently with a mouse-drag or keyboard shortcut. You may want to check this before anything else
In the event the above is not the problem, you may have a bad right channel output when using USB only in which case I'd suggest taking the laptop to a certified Apple repair technician and having them test it...
Write me an email if you want to talk about this further...