[Kees Veling]

From the FCP manual:

https://documentation.apple.com/en/finalcutpro/usermanual/index.html#chapter=26%26section=4%26hash=apple_ref:doc:uid:TempBookID-ReplacedWhenAssociatingWithMessierRevision-44032IMP-1010395

quick fix: add the audio to a video clip in the right frame rate in quicktime 7 pro. Export as reference movie. Import into FCP. Tadaa! audio is of correct lenth

Drop Frame Indicators in Broadcast Wave Files

There is no ratified standard to indicate or detect whether a BWF file was recorded with drop frame or non-drop frame timecode. Although there are several de facto ways this information is stored, Final Cut Pro does not support reading this information. Instead, Final Cut Pro always calculates non-drop frame timecode for BWF files. You can enforce drop frame timecode during import or by using the Modify Timecode command. For more information, see Importing Broadcast Wave Files.

Mapping Audio Samples to Video Frames

Timecode is calculated for digital audio files by mapping a certain number of audio samples to each video frame. When the sample rate is an integer multiple of the video frame rate, the timecode count can be accurately mapped to the sample count. For example, if the audio sample rate is 48,000 Hz and the video rate is 25 fps, every 1920 audio samples is equivalent to one video frame.

However, with NTSC-related video rates, there is no simple relationship between the number of audio samples per second and video frames per second. To avoid ambiguity, the Society of Motion Picture and Television Engineers (SMPTE) specifies how many audio samples should be mapped to each video frame. The number of audio samples per frame is varied on a frame-by-frame basis to form a consistent pattern. Instead of an exact number of samples per frame, a consistent pattern of varying samples is spread over several frames.

When you import a BWF file, Final Cut Pro calculates timecode based on two parameters:

The editing timebase (frame rate) of the currently selected sequence preset

The audio sample rate of the imported BWF file

When the sequence preset frame rate is 29.97 fps and the sample rate of the BWF file is defined in the SMPTE specification, Final Cut Pro uses the repeating frame pattern defined in the SMPTE specification.