Music Creator 6 Recording over SI Drums

Ok, well what I mean by how you are interfacing is, what you're using to go from your guitar to the computer directly, then from the computer to the speakers directly. It sounds like you are using no audio interface, and that is almost certainly part of the problem.
 
An audio interface is basically a box which you can run everything into, and everything out of--in other words, one driver for in and out.  For instance, I use a USB Dual Pre which has stereo 1/4" and stereo XLR ins, and the stereo 1/4" and stereo 1/8" to my speakers/headphones. So let's say I'm recording a vocal track. My mic is hooked into the Dual Pre, NOT directly to the computer (even though it does have a USB connection, but I don't do it that way because of the exact problems you're describing), which then runs USB to my computer. Then, to hear the sound back, it runs through that USB back to the Dual Pre and from there I have connected my speakers.
 
So, when I go into preferences in MC6T, I have my Dual Pre as both the Recording and Playback device.
 
Do you follow me on that?
 
I can't absolutely, beyond any shadow of a doubt, say that your problem is in this area, but the switching back and forth of drivers, and the problems you're having getting everything to work together nicely, it is certainly very likely to be that you are setting your input and output devices to two separate things, rather than one "hub," if you will.
 
I hope that makes sense. My bets are on that being the first thing to look at to resolve the problem.
 
If everything I just told you, you already know, I do apologize. But from your description of the setup, there is no Audio Interface in the story--that is, not one that is a two way interface built for this purpose.
 
PS: Here's a video about recording audio. This is in MC6, but it works the same in this regard exactly as MC6T. The first thing he does is show the interface and how it works.

It has nothing to do with SI Drums. It is strictly a Record Mode/Driver issue.