It is very likely that the TV is preprogrammed so that the sound stops coming from the speakers when a device is plugged into the dedicated earphone jack. This is programmable on many later TV models so that you can change the settings yourself. As for sound quality, TV broadcasts and video content are usually not at the same higher level of audio quality as most music recordings, so you may not notice any difference whatsoever, even with a greater cable length. There are of course many exceptions, and you may find a slight loss in volume. I would suggest using the highest quality cable extension you can find - but at a reasonable cost. You generally get diminishing returns for every additional dollar you spend on cables over standard cable costs.