When it comes to sound compression, the DSP Group's TrueSpeech codec is about as small as it gets. This codec reduces the signal to 8,000 Hz, in mono, with a bit depth of 1. This means that the sound goes through at a rate of 1K per second--an eighth the size and rate of the smallest PCM audio files (like WAV and AIFF files) and a quarter the size of the smallest ADPCM files. Of course, they make everything sound like a so-so quality pay-phone call, but they're certainly small.