Do all sliding glass patio doors come pre-assembled in one piece?
A swinging door would be very convenient in this case since you can very easily find a thick door, seal any gaps and install sound insulation. However, if you really won't be able to install a swinging door you might find this helpful:

Door:

You could install a soundproofing sliding door, with roughly the same money. I suggest installing above the door entry a slider. Now you can find a really cheap wooden sliding door that fits around the turn. You don't need to go for a glass door, it is more expensive and will be more difficult to soundproof. A swinging door will do just as fine if you tweak it and add some rollers on top of it. This is what requires the most effort because you need to be very precise with the measurements, in order to leave as small gaps as possible. (In fact, we did the same on a door we couldn't replace because it was too old, and we weren't able to find doors with those dimensions).

Soundproofing:

If the door is really cheap, then it is also probably hollow and that won't block the sound. You can install soundproofing material of your choice (you can find really good deals for old foam mattresses which do the job) on the side of the door that is always shown whether it is open or closed. Lastly, however good the measurements are, there will always be gaps on the sides. To eliminate this, you can glue some sort of soundproofing strips on the 2 walls on the side, the ledge above and on the bottom part of the door, so that they are a tight fit for the door.

I hope this helps!