I'm still vaguely searching for a good free spectrogram-to-sound program, but this Sound to Graph to Sound Java applet is a good start.
The default resolution is pretty low, but if you're not interested in any of their demo sounds and just feel like doodling, you can hit the reset button to adjust some of the parameters. You can increase the resolution to MxN = 448x448, and to get the frequency bounds closer to vocal range I'd set FL to 10, keeping FH at 4000 (or possibly changing to 8000). You can also increase the value of NFRAME, which lets you store multiple sounds which you can navigate through with the arrow keys, or check the animate box to play them in sequence.
Mostly so far I have succeeded in making lots of drawings that all sound like frogs. Curious.