I have an Early 2008 iMac with 4GB of RAM, running OS 10.6.8. There is a problem with sound breaking up. I am not sure if it is hardware of software related however it seems to be more prevalent with video streaming on the internet than with a DVD.

I have run disc utility repair disk but with no improvement.

If you have earbuds or headphones to plug in as alternative for the built-in speakers, then you should be able to determine if it is the speakers or the signal.