I have been unsuccessfully trying to find an answer to this one for months, and finally came up with a simple solution last night. Thought I'd share it to help anyone else trying to do the same thing.
The problem: All I wanted was to allow Record 1.5/Reason 5 to record from other applications on the PC. I could record my guitar via a Line 6 UX1, I can also record my voice via a microphone plugged into the same UX1, and with the microphone, I can sample any external sounds, but I just couldn't find a way to wire up the thing to record from say a sound file embedded in a web page, or another online sound source. I wrote to Propellerhead and they advised me to look up this thing: Jack. After reading through the site a few times, I decided that unless I wanted to become some sort of low-level, command-line audio technician geek, I would never be able use that utility.
The solution: Then it struck me - why not run a lead from my sound card (UX1) headphone output to the guitar input (or I could have used Line inputs). Voila. All I have to do is run the UX1 software - Pod Farm 2.0 in this case - and select the input I am using to route the headphones, then in Record 1.5 I just create an audio channel and there you have it! Press record and you're off.
Then today, I also found this: Freecorder. Lets you record from software and then use the files for whatever you want.