Reply To: Compressing Talk and Podcasts to a CD
Going mono only halves the size in uncompressed formats. MPEG for example notices that the signal is the same in both left and right channels and encodes it only once.
You can also reduce the sampling rate and bit depth for speech. For example, the PSTN has a raw rate of 64000 samples per second and 8 bits each, except the μlaw codec makes the samples seem as if they have approx. 12 bit range (if they were done with straight linear PCM). A popular format is 22.5K 16 bits, which is fine for speech. 48 kilobits per second for MP3s is usually plenty for speech.