This has been happening for some time now, but it's to the point where I don't want to even get in my studio and play anymore, so time to address the problem, I guess. Problem is, I can be working in SONAR, and then sometimes the audio will speed up. The only way to get it to stop at that speed is to close SONAR and restart it. It always happens if I play audio from say the Internet (i.e. YouTube), or from Windows default music player (or probably any music player) and then go back into SONAR. So, I know that's a trigger for this problem, but lately, it seems to be sometimes going into fast mode without playing sound from some other source. So, I really don't know where to begin with fixing this problem. My soundcard manufacturer suggested it sounds like a potential sample rate issue. They suggested check that everything is set to the same sample rate. But, isn't the sample rate set in the soundcard, and if everything is playing through that same soundcard, wouldn't that be a moot point?

Yes, it sounds like you're running SONAR at 44.1kHz, and generic multimedia apps are switching the interface to 48kHz. SONAR used to throw warnings when this happens, but seems to be more tolerant of sample rate mismatch than it used to be, leaving it to the user to notice and correct the problem. But that's Platinum, and it seems you're still on X3...? I always got warnings from X3 when the sample rate didn't match and SONAR wasn't able to force the interface driver to change rates (e.g. due to being locked to an external clock).
 
In any case, the best solution to this issue would be to have a dedicated audio interface for SONAR, and let generic apps use your onboard PC sound system. Barring that, you can disable "Share drivers with other programs" under Preferences > Audio > Playback and Recording and/or consider running SONAR at 48kHz for new projects so that the sample rate is consistent with other apps.

Thank you. I'll try upping SONAR's sample rate to 48k and see if that fixes it. I already had the "Share Drivers" unchecked, so I'll leave it that way for now.

The sample rate is not fixed in the sound card each application is free to poke it's desired SR in the sound card if supported by the sound card that is.