You have installed k3b with mad and ogg encoders for example, but when you want to burn an audio cd from mp3 and/or ogg sources this message appears "error while encoding audio tracks". However, all in k3b was Ok, you have the encoders required working.
The problem is for an unknown reason in the last dialog box before starting the burning process whereas the k3b Temp Directory didn't appears so k3b audio cd mode hasn't got temp folder to work, it seems to be a gui problem because when you slide around the writing modes (raw, dao tao, etc) the temp section appears mysteriously then you change the writing mode back to auto and choose a suitable burn speed and the writing process starts successfully.

Well really this silly solution works for me and I think that it should work for you even and when you have the encoders required installed ( go to: k3b / Settings / Configure K3b / Plugins and see what encoders have you got)
and also go to Misc in the K3b Configure section and enable the Check System Configuration option.

good luck! and let the force be with you