Hi,
Long story (pretty) short answer.
There is no rate! at least not in the audio timecode. Make sure what 29.97 the recordist used - DF/NDF.
FCP 5.1.2 will use the "startup setup" - means when it was closed last time with 24 fps it will use 24 fps. So if your project was/is 24 fps, audio tc64 will be interpreted at 24 (or 23.976 or 29.97 NDF or 25) depending on the current setup of yours when closed FCP). So you can't import 25/29.97 DF/29.97 NDF/30 DF/30 NDF audio into a 24 fps project with the correct timecode.
29.97 only works if audio tc is 29.97 NDF audio - not 29.97 DF audio!
You need to convert those files into QT movies with timecode. Use the free Sebsky Tools or my expensive BWF2XML (forgot that: it's available at the LAFCPUG shop).
Or start a new project with 29.97 NDF audio (but only in case the audio is 29.97 NDF - DF doesn't work) then open your 24 fps project and copy/drag the audio files from the 29.97 project into the 24 fps project. Using the latter method your audio files should NEVER! go offline during the project, otherwise they will be interpreted at 24 (or any other fps which was choosen when FCP closed) again when reconnected - or you do the above procedure again.
Not that helpful but anyway -
Regards
Andreas