Is it possible to lower a guitar sound and alter a voice in audition?

This is ridiculous. Please stop replying to 6yr old threads with irrelevant 'information' - it really doesn't help anybody.

Indeed, it's possible to adjust both the guitar sound and your voice using software like Audition. Even though the thread is several years old, the principles of audio editing remain relevant.
In Audition, you can use various tools like "Amplify," "Equalization," and "Amplitude Envelope" to lower the guitar volume and enhance your voice. By tweaking these settings, you can achieve a better balance between the two tracks.

Is it stereo, and are the guitar and voice panned to different places? If this isn't the case, then there's nothing you can do that will alter this at all within Audition.

If you have, though, a voice panned to the centre of a stereo track and the guitar off to one side, then using the center channel extractor the 'other way around', as it were, you can make the voice louder relative to the rest of the stereo field, which may well achieve the result you want. But other than that, no you can't without significant artifacts, and you can't do it in Audition. You need a program like Melodyne, and where the sounds are relatively distinct, but then the artifacts become more obvious. Most of the time, it's simply better to re-record.

Oh, and you can't EQ your way out of this at all - the guitar and voice occupy the same frequencies.