Swollen battery, sluggish SDD, no sound - an original Mac Air was dying.
Rather than buy a new one I wanted to try and breathe new life into a classic.
Pretty good, great instructions from the site, and the new sound card was perfect. Sadly, when taking the sound card out I discovered that the cable was also damaged - probably by the b**gers that I had taken my Mac to several years ago when the problem first began. Still, gave it a good clean and installed the new sound card - next up I will be getting a new cable and possibly a speaker too (if I'm going to all the trouble again I think I might as well).
Changed out the SDD for a lager capacity (64 OEM Samsung to 128GB SDD - from another site as you don't have the ZIF SDD's) and also changed the battery too (from another site as you were out of stock). Clean install of OSX and it is as happy as a clam.
Be especially careful with the screws from the battery - my battery had swollen and I had to use pliers to get one of the screws out - luckily there was no damage, but that screw was ruined.
Go slowly and carefully or you will forget something.
I found it very useful to place the screws on a clean surface next to the machine in the order that they come out - it makes re-assembly easier. And get the tools from ifixit that they suggest when you buy - it seriously improves your efforts.