Faint audio output to headphones

I am experiencing an issue where the sound output to headphones is extremely faint.

Whereas setting the OS X output volume to max normally makes earbuds sound as loud as speakers, I can barely hear music at that setting, even with iTunes' volume all the way up too. (I would describe the headphone volume as what you'd normally experience at one "click" above mute.) In addition, jiggling the headphone plug sometimes results in a loud (i.e. expected level of volume for the setting) popping sound, but it is not repetitive and only happens when the plug is moved.

I have verified that the headphones I am using work on other laptops and mobile devices, including a 13 inch Retina MBP. I also found that the Sound panel in System Preferences checks out, with the laptop being able to detect the presence of headphones and that the sound is not muted. There is no red light coming out of the port, and there is no visible dust either. I have also reset the PRAM but to no avail.

The strange thing is that the internal speakers are as loud as ever and exhibit no static or popping noises, and besides this annoying audio bug no other problems have been associated with this MacBook.

I appreciate any insights to help solve this problem!