Just curious....I'm encoding some spoken-word CD's.

From the same mono source, is a 64 kbps mono file the same quality of a 128 kbps stereo file?

Thanks!

No. From a mono source a 128 kbps joint stereo file will have about twice as many bits available for encoding as will a 64 kbps mono file. On the other hand, a 128 kbps dual stereo file (in which exactly half the bits are available for each of the two identical signals) will be the same quality.

Edit: I didn't notice that you were referring to AAC. My answer applies to mp3, but may also apply to AAC.

If you put completely identical stereo in both channels, a good AAC encoder will spend just short of twice the bits if you encode at 128kb/s for stereo instead of 64kb/s for mono on a signal effectively monophonic. So it will be a lot better.