I have audio from a whole lot of different sources and some of it has been converted from wav, some from aiff and some from mp33. I’ve just realised that in audacity all these three types have been recorded at different sample rates - 48000, 44100 and 22050 respectively. But I need to have them all in the same project. Is there anything I can do? Can I have different sample rates on different tracks or will they be messed up by the overall project sample rate (shown in the lower left hand corner).
Thanks 
Use Audacity 1.3.12. The old 1.2.6 version cannot handle multiple sample rates in the same track and has some other quirks regarding mixed sample rates that I don’t remember off hand.
If you want your final exported project to be at 44100 or 48000 Hz sample rate, import one of the files and check in the lower left corner of the Audacity window that the “Project Rate” is set to the sample rate that you want for the project. If it is not set at the rate that you want the finished project, click on the Project Rate and change it to what you want. The first file imported into a new project may change the sample rate, but once the project has at least one audio track the project rate will stay as you have set it. If you import other files with different sample rates they will play at the correct speed as Audacity 1.3.12 will automatically resample them on-the-fly during playback (the actual sample rate of the data is unchanged).
When you Export your finished mix, the exported file will be at the same sample rate as the Project Rate (as shown in the lower left corner of the main Audacity window).
It is better to have all imported files at the same sample rate as it requires less CPU power from your computer, but modern computers should be able to handle multiple sample rates as long as you don’t have too many tracks in the project.
Thanks for this, really saved me!