Ah, the Origin Audio CDs. The name is actually an anachronism: the first CD was "Origin Soundtrack" and the second was "Origin Soundtrack Series." Audio CD Vol. X comes from the third and by far most common release, which was included with the premiere edition of Wing Commander III.
The history behind the first volume remains a mystery today. I've heard many tall tales about it, but I think the MOST likely is that it was actually produced to give out at the 1991 COMDEX show. I /don't/ think it was specifically an Origin Christmas gift. Those tend to be Christmas-specific... and frankly much, much more common (300 copies given to Origin employees who knew their value would show up at auction much more regularly than a thousand given as swag at a convention.) (Similarly, GameBytes' old theory about it being an EA UK release really doesn't hold water at all, since the CD would have predated the buyout entirely... and copies from the UK would have started to show up when game collecting 'went global.')
For many years it was thought to be a myth--something that didn't actually exist.
I actually saw the first known copy discovered. Some years back, Joe and Paula Garrity and I were down in Woodbridge, Virginia to meet with the (now presumably former) owner of X-Toys to buy his Wing Commander movie prototype ships. We handed over the money in a restaurant, got our ships and then went to check out a truly amazing store Joe knew--a massive computer junk shop where you could spend hours and hours just digging through old hardware and games for treasures. The place was run by an old Chinese woman who seemed stern at first but was actually really friendly--she'd happily haggle with you over the price of old electronic trash. She apparently knew Joe quite well at this point as he had been in several times to buy carloads full of old Origin games... and this time she took him aside and said she had something special for him. Sure enough: a copy of the Origin Soundtrack. Five dollars. We couldn't believe it.
Someone discovered a case of them years later and eBayed them for $100 to $300 each a few years later, which is where I picked mine up. I sure wish I had Joe's story!