Salt mines is a description of any routinely dreary programming work. A salmon day is what you get when dealing with marketing men: "you spend all day swimming against the current, and at the end get shafted".
Mung is slightly more complex. Originally used as a safe swear-word in English convent schools in the Thirties - "Oh mung!" - it came to mean, during the Second World War, a sort of corned beef that bore the same resemblance to the real thing as "mung" did to real oaths.
It next found its way to the Model Railway Club at the Massachusetts Institute of Technology, a group of ubernerds who changed the world. They set out to build and maintain what may be the most complicated model railway network in the world, but when computers came along in the Fifties, they started to play with them too, and invented hacking: the activity, as well as most of the words that go with it. These included "mung" in the sense of "break". (About the only words the Model Railway Club never used were "trainspotters", and, despite the Boston winter weather, "anoraks".)
The engineers also produced "friodes", otherwise known as sound-emitting diodes - from the crackle they give off as they let out their "magic smoke". Magic smoke is the stuff that makes all electrical devices work; the proof of this theory is that when the smoke comes out of them, they stop working. (Magic smoke is, of course, an entirely different substance to the vapour in "vapourware", which is a product that doesn't exist except in the imagination of the company selling it.)
The truly amazing thing about computer jargon is that so much of it makes immediate sense. True, many of the things that are meant to be easy to understand, such as "menus" and "dialogue boxes", are incomprehensible to beginners. ("Ah!" cried my mother, when it was finally explained to her. "It's a monologue box!") But anyone can tell from the mere sound of the phrase that a computer that has gone into "mumble mode" is in a bad way. In fact it will soon get a "three-fingered salute" (the combination of keys used to reboot it). Only someone who is totally "404" would fail to see that. (404, most common number on the Internet, is the error message you get when a page does not exist, as any fule find out soon enough.)
It may be that people are making up such stuff simply to get into Wired magazine's on-line Jargon file. If you work all day in a phone farm what else is there to do?
But jargon has always been fun to use, and is constantly being absorbed into everyday language. "Hack" once meant riding gently on horseback. And as recently as 10 years ago, unhappy office-workers once spent all day gazing miserably out of windows - not into it.