Did you try a new drive?
I have an iMac G3 running OS 10.3.9. It works fine, but recently I started having the familiar iMac problem of ejecting CDs. Well, the most recent time, it took me quite a while to dig the CD out. Since then, when I insert a CD, it just slides in the slot. No attempt by computer to accept the CD, no motor whirring, nothing. The only time the CD drive makes any noise is when the computer starts up. It whirs like it's accepting & reading an inserted CD. Otherwise, nothing. Thoughts?