From within Terminal (Applications->Utilities->Terminal)
Mac OS X Tiger (10.4)
You can determine the device that is you CD/DVD drive using the following command:
drutil status
Vendor Product Rev
Type: CD-ROM Name: /dev/disk1
Cur Write: 16x CD Sessions: 1
Max Write: 16x CD Tracks: 3
Overwritable: 00:00:00 blocks: 0 / 0.00MB / 0.00MiB
Space Free: 00:00:00 blocks: 0 / 0.00MB / 0.00MiB
Space Used: 66:55:27 blocks: 301152 / 616.76MB / 588.19MiB

Now you will need to umount the disk with the following command:
diskutil unmountDisk disk1
Now you can write the ISO file with the dd utility:
dd if=/dev/disk1 of=file.iso
When finished you will want to remount the disk:
diskutil mountDisk disk1
Note: You may also be able to use hdiutil create -srcdevice /dev/disk1 -format UDTO file.iso to create the ISO image. I have seen this work for data disks, but have not had much luck with audio CDs.