I cut the ribbon cable at the 40 pin plug for the DVD drive to shorten it to a third of the original lenght. After I did this, I checked no wire was shorting out with a magnifying glass.
DVD drive fits nicely now with a bit more room for airflow.
Installed an external 5.1 sound card, which fixed up the sound quality. I thought I had a fault sound card, until I figured out the left speakers were turned down. Doh!
Speakers still pop when I turn off as the PSU is still dodgy.