Found an excellent audio joiner in this programme MediaJoin 2.0 see (http://www.tucows.com/preview/396569#MoreInfo for more information). It is free and so far has joined several mp3s flawlessly for me for use in making long slide show movies with Picasa. I tried other free audio joiners but none came up to expectations. For example mp3joinsplit http://www.softpedia.com/get/Multimedia/Audio/Other-AUDIO-Tools/MP3-Splitter-Joiner-SH.shtml came with some problems of very significant noise during playback of the joined mp3s. Another program was Direct MP3 Joiner but this turned out to be free for only a limited period.
Why does one need to join mp3?
For one, it is necessary for slideshow producers like Picasa3 where there are limited audio options to go with the slideshows that run more than 3 minutes. Picasa3 only allows for 1 audio track per slideshow -- or movie as they term it-- and there is no audio looping; so if your slideshow is long the music stops before the sequence of slides ends.
Of course one can use Picture Story 3 -- also freeware -- from Microsoft but this has limitations as it is only "pan and zoom" for all slideshow movies. But it does allow you to add audio tracks anywhere along your slideshow movie and this is good is you have slideshows with different segments and you want each segment to have distinct music accompaniment. A good feature of Picture Story 3 is that the audio fades when it is truncated and thus the changeover from one song to the other is not abrupt.