Couple of quick thoughts:
Headphones for mobile devices are a right pain to design, because they're driven by very low voltages.
This means the cable has to be very low resistance, and for practical purposes quite flexible too. And that in turn means it's expensive and hard to terminate (where it joins to the plugs and the earphones themselves). See the Wikipedia entry on "
Litz wire" to see how it used to be done. That stuff CANNOT be soldered in my experience (and boy, have I tried in the past!).
Most headphones break mechanically, either at the back of the jack plug or where the cable joins the headphones themselves. Realistically, they can't then be repaired.
For longer headphone life, avoid right-angled plugs. The straight-through ones that let the plug come out when yanked are best. Better still are the designs with a socket on the headphones too - that gives you two chances before the cable itself breaks.
You can also get short plug-to-socket adaptors (about 10cm long). These are great, because a yank on the cable is always a straight pull on the plug, which it is supposed to cope with. They also save the socket in the device, which can be a very expensive repair if they're broken.
I usually catch up a bight of cable in the cover for my tablet, so that the plug doesn't get tugged immediately if I'm clumsy. It's not ideal, but it helps, partly because the plug isn't being directly pulled, and partly because the cable isn't tugged in exactly the same place every time.
Also avoid forcing those cables round sharp bends, especially coming out of the back of the plug.
I hate designed-to-break products, but we're mostly stuck with them nowadays.
And no, using a short adaptor cable shouldn't have any noticable effect on the sound quality, unless the connectors are actually not working properly or are really, really poor quality.
HTH,
E.
