Ok. I am not a total newbie to recording, mixing and mastering HOWEVER--I am totally lost right now...

I use Acid Pro as my DAW--and try to master everything in WaveLab or Audition (depending on my mood)--however something is happening that I have never seen before.

My mix sounds pretty good (not great) in the headphones, but like HAMMERED MONKEY CRAP on the monitors. I thought maybe it was the monitors or the soundcard, so I burned it to a CD and played it in the car and it sounded TOTALLY different. Not as good as the cans, not as bad as the monitors--

But here's the kicker--the difference is that on the Monitors and in the car, the vocals sound flat--but NOT in the headphones--I have re-cut the vocals three times and while recording, they sound great--but on playback they sound flat, but only in the monitors--

WTF???
Any guesses?

Dude, your "panic" title isn't going to endear you to anyone.

That said, you're not telling us what your monitoring chain looks like - what kind of cans you're using, what kind of monitors, amplifier on each, DAC, etc.

Even without that info, the stereo spread is always exaggerated in cans, which makes anything panned center seem bigger than it is.

How are you doing your mixes?

Through the cans or the monitors?

Ok. Well I guess I am sorry for that, but I didn't realize that there were rules on titles. I was trying to be funny.

To answer the other question. I am doing my mixes primarily through the monitors but go back to the headphones from time to time, the same way I always have. My headphones are AKG 271's and my monitors are Mackie SRM450 Actives.

This is not a new set up. And this has never happened before. And I did a new mix last night of a different track and didn't have any issues at all...but came back to this one re-mixed it and had the same problems all over again.

Maybe you should check your settings on that one "bad" mix.

It is possible some kind of unexpected processing is being applied...something that you are not aware of...?

I never mix with headphones anymore. I might use them when I am tracking with a microphone but I already suffer from mild tinnitus and I certainly don't want it to get any worse. The last time I tried to mix with head phones I spent at least a couple of hours going back and trying to fix things that came up after listening to it on a couple of different systems.....I had gone a long way down a road and it was really disappointing to have to go back in and tinker for so long after thinking I was done. While I am on the subject, I found that using Izotope Ozone for mastering really helps me finalize my stuff since I don't exactly have any acoustic treatment. The EQ section has about 16 different acceptable EQ standard curves (and you can add your own from your favorite CD as well) and you can do some nice tweaking by watching your own mix in comparison/playback.....I know this is kinda archaic since you are supposed to be doing all this stuff with your ears but it sure has saved me some embarrassment when I try to give someone a CD and have to sit there and listen to it with them on their system.

http://www.myspace.com/mortal_engines

From what I am hearing here is your Mixes are not transfering well. Thus, you should look into treating your Control Room area (see Ethan Winer's Acoustic Forum). Then you best be looking at just what kind of Speakers you are Mixing on because at this point you are unable to trust anything you are adjusting/Mixing.

I thought it was funny.

Bottom line - my opinion is your speakers are garbage. We used to listen to our final mix pretty much flat using car stereo speakers to get the sound away from the studio boxes. Headphones will always sound better but it won't sound the same as on speakers.

Mixing with headphones is a very bad practice, and Mackie SRM450s are not control room monitors they are live PA on a stick speakers, not in any way designed for studio mixiing. Ditch the phones, get some real monitors and start again.

"--the difference is that on the Monitors and in the car, the vocals sound flat--but NOT in the headphones--I have re-cut the vocals three times and while recording, they sound great--but on playback they sound flat, but only in the monitors--"

Obviously, the difference is in the response of the monitors, the car, and the headphones. Now, your statement is contradictory...
"..on the Monitors and in the car, the vocals sound flat.."
"..they sound flat, but only in the monitors--"
so which is it? Flat in the car and on the monitors, or only on the monitors?

If the vocals sound really good when you cut them, and less punchy or in your face when you play them back, then there are two choices:

1. you are hearing a boost in those frequencies in your headphones. So you don't push them when you mix. So when you play back the mix, those frequencies are weak in the monitors (and, possibly, in the car.)

2. you are hearing a hole in those frequencies in the monitors (and, possibly in the car) which corresponds to the punchy frequencies of the vocalist.

Whichever it is, two things said here remain true... Mackie SRM450s are not a monitor system (which includes the room, room treatment, speakers, amps, attenuators, converters, placement, etc); and headphones suck for mixing.

Vocals always sound louder in cans, because they are centered in the mix, and cans are always going to have a more extreme stereo spread than regular speakers - i.e. if something is panned far right, you'll only hear it on the right side, not blended in the room field.

Hence, vocals always sound more present in cans than in real speaker applications.

In short, turn your vocals up until they sound right in the room. The cans lie.