I own the 1992 edition, which has been very useful. The latest edition is the '92 edition repackaged; the contents are the same. I confirmed this with the publisher. The price has dropped merely because the packaging is cheaper to produce; that's according to their editorial staff.
If you wish to use the Peterson CDs "in the field" with a portable CD player, or if you use it frequently at home, there is something you should know:
Peterson organizes the songs into two CDs, each with about 50 tracks. Each track contains from 2 to 8 birds. If you want to hear a specific bird without hearing others, you can go directly to that bird if your CD player has the INDEX function. An index is like a subtrack.
Herein lies the problem; most portable CD players no longer have the INDEX feature. It was phased out years ago. Thus, you have to listen to all the birds of a track to hear the one you want, and as you get better at birding this becomes tedious.
It should be mentioned that most CD players do have a "search" function which allows you to "fast forward" through a track. But this is awkward to use.
Some makers of bird song collections on CD (e.g. Stokes, new out in May 99) have learned of this problem and have corrected it by recording one bird per track. Stokes also seems to have clearer and longer recordings. However, this format means that the full collection requires 4 CDs. Thus, using Stokes in the field has its own problems.
If you can find a portable CD player with the INDEX function, the Peterson CD works well, especially the Eastern Birds, which is one-volume. I can just plop my portable CD player into the pocket of my field vest, and any time I want to confirm a warbler species I can cue up that bird in an instant. I can quickly INDEX through the birds calls and compare. Peterson gives the bird label and the bird song separate indexes, so you don't have to listen to the announcer naming all the birds. But unless CD player makers change their minds, this convenience may die when my '89 Sony portable does.