Are you talking about being able to use the "CD" (that you burned onto a DVD) in a regular CD-ROM drive or car stereo? Because it doesn't quite work that way, CD's and DVD's are physically different media. The "Guiding Track" used to keep the burning laser (and then the reading laser) on-target is much, much, tighter on DVDs than on CDs (something like 5 times closer together).
You can burn an ISO9660 standard *.iso file to a DVD (with joliet or RockRidge Extensions), but you're going to need a DVD drive to read that burn.
As for audio, you could try to find something (DVD audio tools
, maybe?) that could burn a "DVD-A
Keep in mind, none of these solutions let you put a burned DVD-R into a regular CD drive.