The sound output port on boards version 1.0 and 2.0 are miss-wired causing distortion and a DC offset which could damage certain types of DC-coupled amplifiers and even loudspeakers. It is recommended that you use the HDMI or I2S functionality. If you need to use this port, make sure that the equipment you use has a DC blocking capacitor. The specific problem is that the surge suppression network, which should come between the SoC and the DC blocking capacitor comes AFTER the capacitor, creating a rectification effect of the signal, causing a DC offset and distortion. The design fix is simple, move the DC blocking capacitor after the surge suppressing diodes and then add a 100k resistor (470k, or even up to 1M is fine too) to ground. A series 100 ohm resistor just before the jack might also help suppress unwanted RF interference. If high quality output is required from existing boards, it is necessary to remove parts D12 and D13 from the back of the board. The three pads should be VCC, signal and GND respectively, identify each pad and carefully install some resistor between 10k and 100k between the signal and GND, making sure not to short out VCC. This is fairly high-end SMD re-work, I would suggest you get some training, tools and practice before attempting. (ed note: the writer of this comment does not have said skills, only knows what would have to be done).
