I have recently installed Ubuntu 11.04 on my Macbook and after a few days I am now getting very low sound from the speakers as well a red LED is glowing in the Headphone jack.
The sound is perfect if I connect my headphones but if I use the speakers there is no output. I have also installed Windows and Mac OSX(Triple Boot). The speakers work fine when I use either of them as well as there being no LED.
I have re-installed the Ubuntu but then also the problem is not solved.
[FIX] : Big thanks to Vonhinten I met him on irc #ubuntu who helped me fixing this issue :)
To get the sound i added : options snd-hda-intel model=mbp55
to the file
To turn the Red LED off - try: amixer set IEC958 off
This fix was also featured on OMG! Ubuntu : http://www.omgubuntu.co.uk/2011/05/how-to-fix-low-sound-on-a-macbook-with-ubuntu-11-04