If you control-click an audio track with a linked clip in Final Cut Pro 5 and choose Send to > Soundtrack Pro Audio File Project, you may not be able to select it if it is dimmed. This happens if you are working with linked clips from different media sources.
To send audio from a linked clip, both the audio and video have to be from the same media file. But even if the audio you wish to send is from the same media file as the video, there may be additional audio tracks from other media files linked to the clip. In this case, you can't send to a Soundtrack Pro Audio File Project.
You can work with audio from a linked clip by either unlinking the media in the clip before sending to Soundtrack Pro or by choosing Send To > Soundtrack Pro Multitrack Project instead of Send to > Soundtrack Pro Audio File Project.
Note: This can also occur if you link two mono clips to create a stereo clip.