It's a 386SX, which absolutely will run Windows 3.1 but if you're going to be real picky, it definitely won't be as snappy as in the video ;)

Question is, would you be able to put a Soundblaster 16 inside it?

Tandy 3-voice can play midi without an add on card.

Right but I know Yamaha OPL sound when I hear it.

The Tandy 1000 RSX has several ISA expansion slots. Putting a Sound Blaster in it is no problem.