I don't really understand what you mean. Do your M50's sound poor whatever they are plugged into now? Sounds very strange what could have caused it.
How can something touch your headphone jack when it is plugged into your laptop? When you say the jack, do you mean the metal shielding further back. Sometimes, but now often you can get interference but I don't know what your problem is.
If they are under warranty, maybe contact Audio Techinica. If they are not under warranty any more, maybe try rewireing them with a new plug.
http://www.maplin.co.uk/p/35mm-metal-stereo-plug-fj99h
I have used this sort of plug before and it should work fine with the M50s cable. You will have to read up about how to do this though because I don't know enough about it to explain it to you well.
If you are not at all experienced at doing anything like this, maybe don't attempt it.