It may be your burner which is the problem.
I have a Lite-On; it ignores the burning speed I have set and insists on burning at least 14x.
I've tried it with five burning programs, three Linux and two XP ones. None of them will make it burn at the speed I want.
There is the occasional 'coaster' it produces, but any iso's I burn with it, will only play back on that drive and not on my CD/DVD read only drive. It's very noisy as well.
Thinking of getting a new, better behaved, burner!
The burner is fine Keith, and I could select burn speed with Brasero but see above why I dont want to re-install.
Actually the iso's I have burnt at 12X and 16X have worked OK, but I think that you should have the flexibility within k3b and nautilus burn - right click option on iso files - to choose a speed at which to burn, after all speed isn't everything unless you are Jeremy Clarkson !
Maybe I will try Brasero again if I upgrade from Elysia to Felicia and see if it works for converting mp3 to audio CD.