The following error is common when K3B (CD/DVD Burning Software) is run for the first time on a stock-install system:
Mp3 Audio Decoder plugin not found.
K3b could not load or find the Mp3 decoder plugin. This means that you will not be able to create Audio CDs from Mp3 files. Many Linux distributions do not include Mp3 support for legal reasons.
Solution: To enable Mp3 support, please install the MAD Mp3 decoding library as well as the K3b MAD Mp3 decoder plugin (the latter may already be installed but not functional due to the missing libmad). Some distributions allow installation of Mp3 support via an online update tool (i.e. SuSE's YOU).