Poor Mic & Speaker audio via Cinema display

Skype & FaceTime suffers the most when my 24" Cinema Display is attached. The levels are well down and intermittent. Visually the images are perfect.

If I connect an HP LED display my MBP frequently rejects it and cuts out by reverting to black screen, with and without mirroring.

I've reset permissions, PRAM and SMC. Any clues please? Any parts need replacing and if so at what cost and can iFixit help?