Hello Gary
Link [1]
Link [2] (may be of interest)
According to link [1],
"MP3 could not be opened? If this is the first time you’re burning MP3’s to an Audio CD, it could happen that Brasero complains that certain plugins are not installed yet. A dialog will popup guiding you through the very easy installation mechanism Ubuntu utilizes.
Simply follow the steps. Make sure to check “GStreamer extra plugins” and not “gstreamer fluendo” (fluendo is not compatible with Brasero)!
You can use Synaptic to find and install the gstreamer plugins.
According to Xfburn (another burning tool I would recommend if brasero an k3b still do not work for you after these steps), "The gstreamer transcoder uses the gstreamer library for creating audio compositions. Essentially all audio files should be supported given that the correct plugins are installed. If an audio file is not recognised, make sure that you have the 'good','bad', and 'ugly' gstreamer plugin packages installed (again use Synaptic to install these, enter the search term "gstreamer" and scroll through the list).
So install these packages as well and have another go. It would be best to have brasero and k3b closed during the install.
Try burning with brasero and k3b again.
If both of these still fail to burn your mp3 files to a cd you may want to install and try "Xfburn", again using Synaptic.
If you have the patience try these suggestions, and let us know how you get on. 