Atsuko,
Q1) Why do we have to turn off ARC when we're in the process of mixing down?
Answer - If you set it up like ptheisen, then you won't have to shut it down. The software is just there to "fix" any room issues you have. If you had it on the master bus, then your mix would be eq'd for your mix/recording room only. But set it up like ptheisen and should work fine.
Q2) To simply listen to a song, will we have to disable ARC or is it ok to have it turned on all the time, except when mixing down?
Answer - sure, however you may want it on all the time it won't hurt anything.
Q3) During measurements, the air conditioner can be on?
Answer, If the A/C is in your room, you should turn it off until you are done measuring, if the A/C is in another part of the house, then you should be fine.
Q4) I have an audio card (M-Audio Delta 1010LT) that doesn't have phantom power but I have a mixer Behringer Xenix 1204 that has. Do you think That I can loose something during measurements because I'll have to intermediate the audio card with the mixer?
Answer, maybe, but the software should get you close, I would set everyting to 0 on the eq section of the mixer. Again this tools is to "fix" your room EQ, it does a very good job, but it can't fix it 100%. so user reference tracks when you mix!!!! I am learning that as we speak, it helps alot!!
I hope I helped.
T