Get a multimeter. Put it in the "DC Amps" range, highest setting. Plug the leads into Amps and Gnd. Hook the Amps lead to +12V from the battery, hook the Gnd lead to the +12V to the CD player. Play something at your average listening level with all 4 speakers running. Check the meter, you may have to turn it down to the next-lowest setting to get a good read. This will tell you your amps, from there it's straightforward to figure amp-hours.
There's really no better way to do this.