Understanding Music

When I placed a 48 MB MP3 (45 minutes worth) in one of the 6 folders I
had on my VCD configuration it fills up my disk at 11 GBytes during
slide creation. Before adding music the slides show (including the PC
.EXE file) already used 589 MB.

What format is the sound converted to? How can I calculate how much
sound + pictures I could fit on a VCD? (already calculate that my
cameras stills come out to around 1700 pictures per CD at 4 seconds per
slide).

Thanks,
D.