I have a pair of headphones that I like to use for night time so I don't disturb other, they're AMGs and pretty good, so even during the day time I sometimes like to use them on well produced Vimeo productions for example as they reproduce better sound and you easily pic a lot more things up that standard iMac speakers miss. But, it's a hassle. I have to plug them in round the back every time, and of course un-plug them to get sound again. It seems like it's a hardware thing. I would like to leave them plugged it but either have some software that will auto route e.g. YouTube video to the headphone and have everything else play from the iMac's internal speakers, or have it manual configurable from software, or even a physical splitter, but my feelings are that once the 3.5mm jack is inserted then the Mac automatically stops any sound from ever coming through. Is there anything could do to make this happen please?