During the winter feeding season in Maine, it is too cold to have the windows open, and so I miss hearing the sounds of the birds at my feeder. I have found a solution, however.
A baby monitor, placed on or near my feeder broadcasts the bird chirps and chips into my kitchen. Most battery-powered baby monitors are made up of two separate elements: a monitor unit with a microphone in it that picks up the sounds of baby (or bird) and a receiver that broadcasts the sounds through a speaker simultaneously.
Baby monitors are not terrifically expensive. I found a used one at a yard sale, and thus did not feel at all guilty about leaving it outside in the cold and snow.
One interesting side benefit of using a baby monitor in this way is that I can record the actual bird sounds while videotaping the activity at the feeders outside my kitchen window. Summer feeder watchers can also listen to hummingbirds at their feeders.