In MC6, I have been working with a project for a while, using an M-Audio Fastrack Pro. No glitches until now. I just re-recorded a vocal audio track and when I go to mix, I do not have control over the interleave button, ie, it is stuck ON. I'm only getting sound out of the left channel... weird. Anyone else seen this before and is there a something I can do other than deleting the track and starting over?

PS. On all the other tracks, the interleave button works normally and I can switch between stereo and mono.

You probably recorded the mic on a stereo track instead of a mono track. The "only getting sound out of the left channel" is a common result when recording a mono instrument or mic on a stereo track.
Try Bouncing the track to a mono track, then delete the original track.

I will give this a try... I have been recording on this track and the button has been functional up until now, don't know what I did...

I forgot that MC 6 has a 'convert to mono' function that will probably work quicker than Bouncing.

I converted all the clips to mono and that gave the desired result... Thanks Greg. Cant explain how I got the problem to begin with though! I might have had the stereo/mono button on the Fastrack pro in another position. Do you recommend recording all audio tracks in mono? Bill

yes, unless you're recording a STEREO SOURCE.

mics are mono
guitar DI's are mono
guitar FX processors are typically stereo (consult your manual)
keyboards can be either depending on how you wire it to your soundcard.  if you use both outputs to both m-audio inputs, use stereo, if you use only left/mono, use only mono input on the m-audio
bass - mono unless you're using a bass fx processor and then it depends on the output of the processor

etc.