I have a Macbook Pro (late 2012/early 2013) running OS X 10.9.4, and it's recently developed a strange problem when I try to play sound through headphones. Instrumental music generally still sounds fine, but speech is so distorted it's impossible to understand, and the vocal line on songs is sometimes OK and sometimes distorted or completely absent. Instead of intelligible speech, I get odd high-pitched tinny electronic noises. This applies to anything played within my browser (YouTube, Soundcloud, internet radio etc.), streaming apps such as Spotify, and music stored on my computer played through iTunes. It doesn't appear to be a problem with my sound card either, as everything sounds completely normal when played through the built-in speakers. Neither does it seem to be an issue with my headphones: I've tried several pairs as well as external speakers plugged into the headphone jack, and all of them produce the same problem. I've tried cleaning the headphone socket with a paperclip as someone suggested on another thread, but there's been no improvement; anyone have any other ideas?