If i may suggest some suplémentaire documentation about audio sampling:
Here they suggest that 8kHz is sufficient AND 8bit/mono is over killing for voice recording. So any way's, by the end, following the math I proposed up there, you could quadruple the recording time on the SDcard, so about 16000 minutes on a 4gigbyte SDcard. But at 4bit/mono 8kHz, I suspect that the sound might be very very crappy unless you are a baritone.
