Audio-Out-Jack broken? Sound on left channel much lower.

Hello.

My computer: MacBook Pro 15", Mid 2010 (MacBookPro6,2)

If I plug external speakers in the audio-out-jack the sound is much lower on the left channel than on the right channel.

I already tried to solve the problem by...

- using other plugs / speakers / amplifiers

- blowing into the audio jack and gently moving the plug

... but nothing helped.

I therefore think, that the problem can only be that the jack itself is broken or that the audio software is broken? If the jack is broken, where do I get a replacement and how can I install it?

Thank you for your help!