If the file is on a read-only JFFS2 partition, then you can remount it read-write with the following command (WARNING - be very careful what you do after this as you could mess up your ROM, forcing you to reflash):
mount /mount/point -o rw,remount
and this to make it read-only again afterwards
mount /your/mount/point -o ro,remount
But if the calendar on the SL-1000/3000 is anything like earlier models, the alarm sound is compiled into the calendar binary so it doesn't actually use that wav file and cannot be changed.