How to switch MacBook Pro back to Normal Digital Audio Out from Optical Audio Out?
It's unclear what you mean by &quot;Normal Digital audio out.&quot; Optical digital audio out is the digital audio output if you're not using external USB/firewire/thunderbolt audio, so unless you're working with external audio interfaces switching between optical and digital does not apply as they refer to the same thing. I'm assuming you want to be able to use speakers and headphones as normal?
You've correctly identified that the audio codec IC on the motherboard controls this selection, and although many users find that toothpicks or blowing really hard or a dab of isopropyl alcohol on a toothpick works for them, it's possible for the little switch inside the headphone jack to become nearly permanently stuck. Since the headphone jack still works with headphones plugged in, that rules out the audio codec and means it's a problem with the switch itself. Somehow, headphones take priority over the digital output if both switches are on at the same time.
The solution, which I've described in the iFixit link you referenced in your question, and which originally comes from stin17 on this MacRumors thread, is intimidating but uncomplicated. We simply want to interrupt the line between the audio codec IC on the logic board and the little switch in the headphone jack by removing a component in between. This is usually a small resistor on older models, and after the Retina MBPs I believe it is an inductor.
If you can correctly identify your logic board model number (physically located on the logic board itself, usually a 6- or 7-digit number; for example, my 2012 MBP with a 2.3GHz i7 is a model A1286 and board number 820-3330-B) and find the boardview and schematic, you'll be able to find a resistor in line with the digital output switch, which, when removed, will prevent the audio codec IC from reading the digital switch. This permanently disables the optical digital/TOSLINK audio output, and you should not do this unless you are certain you've found the right resistor! However, if you're sure you know which components you're looking at, it takes about 10 seconds to scrape the tiny surface-mount resistor off the logic board with an x-acto knife.
If you have an A1286 macbook, it is likely R6805, which you can find in schematics and boardviews; this part number will certainly be different on other models. On my logic board, the circuit we want to interrupt is labeled AUD_PORTB_DET_L. Later models seem to rename this circuit to AUD_CONN_TIPDET_1 or AUD_CONN_TIPDET_2; I'd guess TIPDET_1 is headphone detection and TIPDET_2 is TOSLINK detection, but I'm not positive since I don't have one of those boards to check. Make sure you double check these on your particular model! I can't guarantee that R6805 is the one to get, but that's a good place to start.
The result here is that the digital switch inside the headphone jack remains physically stuck, but it no longer matters as it cannot complete its circuit to erroneously tell the audio IC that there is a TOSLINK plug in the jack. Again, optical digital audio output over TOSLINK does not work after doing this, unless you replace the resistor, but it sounds like you don't need optical digital audio output. This is straightforward and familiar for some, for others it's new and terrifying! I started learning my way around schematics and boardviews because of this exact problem. If you feel up to the task and you don't need TOSLINK audio output from your headphone jack, this is the most reliable and elegant fix when the other temporary methods fail.