This may or may not be the solution, but it sounds like you are recording a mono source (guitar) to a stereo track ( which is frequently the default setup in a new DAW) and the DAW is assigning it to the right side. If you only have sound coming out of one speaker, check to make certain that you are recording to mono, then if you are still only getting one side try panning it to the left. Nothing still? Check all cabling, swap out the right side speaker, cables etc. known to be working, check in headphones, if you still don't have anything then the mobilepre is the likely culprit.

don't know that unit but there must be software that came with it to select inputs/ routing and outputs?
when i had an aardvark q-10 interface, after i recorded a track, i had to move the pan slider in the software input mixer/routing to the middle to hear both L/R on playback cause the pans was set up from factory as ch.1 left,ch.2 right and so on. check pan settings on the inputs and also the output routing wherever they might be.
just a thought.

I wouldn't use ASIO4ALL when the asio driver that comes with the MobilePre is sufficient. You'll just be mucking up your computer. A4A was really designed to integrate the WDM or Directsound portions of a motherboard into a DAW. Which is always a very bad idea.

I think your solution has already been noted. Check the pan settings in your DAW and in the ASIO driver for the MobilePre. If you are using A4A then you'll have to check those too but they aren't very flexible.