MacBook – System detects digital output when headphones plugged in, audio does not work

It sounds like a hardware problem in the jack. If you can put pressure on the external part of the plug and change it: cause static or breakup it's probably a hardware problem in the jack.

If you had said you only tried one set of headphones then I'd have said it's probably a problem in the headphone plug but if it's multiple phones and speakers then it's probably the jack.

There's a sensor in that jack that monitors if something's in it or not to switch off the internal speakers. The if there's continuous sound, just switching from internal to external the problem may be that sensor. If the sound cuts out from both internal and external once in a while it's probably the jack.

If the MacBook Pro is still under warranty I'd have Apple look at it, they'll swap motherboard to make it right if it's a hardware problem at no cost to you.

It sounds like this is an electronics issue. Its probably a single wire (the one wire that carries the right channel sound).

There is little that you can do about this unless you know how to solder/feel comfortable doing this repair yourself.

I would suggest checking if your computer is still under warranty and if it is having apple take a look at it. That should likely be covered.