If you KNEW that Audacity records at 44.1 kHz, and you KNEW that FCP likes its audio at 48 kHz, why did you not do the conversion in Audacity or some other audio application before you started work in FCP?
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA
Audacity does an excellent sample rate conversion. Do the 48k convert in Audacity and then reimport new files with slightly different names once you have checked your settings as Nick outlined. Also make sure your bit depth is 16 or 24 bit only. FCP doesn't like 32 bit or float
Great that sorted it. Changing the whole Audacity project settings to 48Khz and exporting didn't change the speed (as it would do when I changed them individually for some reason) and also settings it to 24-bit. Then re-importing to FCP with all the corresponding FCP project settings set at the start and its all good.
The problem you were having with your files speeding up when changed to 48kHz is that you were not converting the sample rate, only changing it. I haven't used Audacity enough to point to where you made the mistake, but the result you describe is pretty clear cut.
In some software, it is a bit too easy to export material at a different sample rate without conversion. This affects pitch and duration, as the same number of samples is being played back on a different time base. When going from one sample rate to another, it's good to look for the word "conversion". That's an assurance the operation will create the missing samples or drop the superfluous ones.
It's the audio equivalent of the difference between adding pulldown or conforming in Cinema Tools to change video frame rates.