Help finding lost music
If you ever synced your iPhone or an iPod to the iMac before the fire, your music ripped from CDs could still be contained on these devices. You can then retrieve these songs from your device to your new computer using a program such as TouchCopy as detailed here
https://discussions.apple.com/docs/DOC-3141

iTunes will only download your purchases to your new device, so if you haven’t synced it before, you may need to rip your CDs again.