So much bullcrap posters
But some are good, so there is hope.
Simple answers
1. Front panel is always worse.
The question remains is it audible? Not always.
It is always measurable? YES, it is and you do not even need pro gear for that.
An the answer is also simple. Depends on the load, PC setup and there is no universal answer. You have to measure not do some philosophy.
1. With low Z load. It may be crucial as you introduce excessive wire length that accumulates a certain resistivity with subpar PC quality jacks, I mean it, those are worst quality ones you will ever see. You will loose power due to loss on the cable, with low Z it becomes a significant percentage. GPU radiated EMI is so powerful you will see it the graphs more when measured. Can we fight using coils, twisting cables etc... on certain degree? - no, the radiated power is way too high and everything is way too close. Simply the needed amount for rectification exceeds the case size. If you use an integrated GPU... then YOLO, do as you wish in most cases. The energy is not enough with low Z to hear distortion in most scenarios and that is the most often use case.
2. With high Z load you risk into driving the output in oscillation is more EMI sensitive. It largely depends on the schematic solutions, but considering the low voltage motherboard audio is capable of outputting as such it is a fundamental problem for high Z loads. In general PC is a place where compromises are done and what fits for all fits for nothing. There are motherboards with dedicated DAC's and headphone amps, those are much resistant to anomalies, but not always, you know there are always Gigabyte boards with ASUS following them closely in profanity race.
Cable has a capacity. If the commutation switch(external or internal HDA) is not properly designed it will act as an oscillator, some even manage to to wire up coils and zip tie them when doing beautiful cable management
. yeah it looks neat, but you ware a nincompoop at physics also. Thing is all tests are done in ideal conditions and without large GPU's. Basically motherboard makers use audio section parts as LEGO bricks and put them on anything, they are not tailored really, so problems may arise due to differentiating current flow in the PCB... Thus why we see different revisions of the same board, especially Gigabyte is known for that(Asus often hides it), some manage to to screw current path on such degree that the it really takes the audio parts as the shortest route also thus introducing audible noises, like hearing mouse movements and GPU revving up like an diesel engine, the signal ground is polluted. We have 300W+ switcher sections like 10-20cm from analog parts... well it is what it is... it is crazy actually.
The use case of front panel audio has stalled in recent years and NO ONE will pay attention to it and will receive no development. In my books HDA header should die. People are already using wireless for years, If not I plug a Type C headphone. The PC case front panel I/O should be be a hub with a dedicated simple audio part, those cost peanuts these days and are decent performers. Case I/O should use only one USB line and that's it, no more clutter and wires, some makers charge us 200$+ these days for a PC case, for what? RGB crap? Put a USB hub, audio/even MIC array and Bluetooth there. Laptops have it, why PC cases should not have the same features and not to rely motherboard have it all and have options, the implementation of it is trivial and would cost under 30€ when doing small batches, on large you can do it under 20€.
