Are you concerned about the effect of playback on birds, given that so many people are carrying digital field guides with audio?
Check out my in-depth discussion of this issue. Playback is a disturbance to birds and to other birders. It should be used as little as possible, especially in popular birding spots and places where other birders are present. It is illegal in many parks and refuges, and it is illegal to disturb any threatened or endangered species. If you do use it, be very sensitive to the birds and to any nearby birders. keep the volume low and play just a few short snippets of sound, then watch and wait.