David Rowe’s Codec 2 packs relatively comprehensible voice into 700 bits per second, for use in the FreeDV amateur radio low-bandwidth voice mode. But what if you record it on an audio cassette tape instead? You should be able to get higher bandwidth than we got out of analog telephone lines — two channels with an ENOB of about 5 with frequencies up to about 15kHz, implying 30ksps. This works out to a channel capacity of about 300 kilobits per second, given the appropriate modulation. (And doing the appropriate modulation should be pretty feasible.)
This implies that each second of audiocassette data can hold about 7 minutes of Codec-2-encoded voice; a 60-minute audiocassette can hold some 400–500 hours of recorded voice.
Fortunately or unfortunately, this is not actually a useful thing to do.