Why won't a soundcard work?
Well the trick with RAM cards is to find one that fits. Most of the ones I've seen are full-length cards and won't fit in the MegaPC.
I have an Intel Above Board 286 card with 2MB on it , and an AST Advantage card with 1.5MB on it. Both are detected automatically by the system BIOS as extended memory and show up on the post screen. For EMS though, they can be a pain in the arse. You gotta track down the drivers and and they can be quite finicky to work. The AST card I have is XMS only, it was not designed for EMS. The Intel card I haven't tried to configure as EMS but there are drivers on intel's website.
As for where to get them, well, same as any other retro hardware.... swap meets, dumps, forums, or worst case - ebay... the lamcsales seller tends to have a few for prices that aren't TOO outrageous.
Here's a link to a couple of EMS drivers for AST cards - REMM.SYS. Be aware with AST cards that a lot of them were made for proprietary AST computers and have an extra connector in addition to ISA (like VESA local bus but a different connector) so won't work in standard PC's.
http://dl.dropbox.com/u/9151127/oldremms.zip
http://dl.dropbox.com/u/9151127/remm470.zip