Matthew, yes, you can hear all audio tracks in FCP as you trim – just be sure to select “trim with sequence audio” in your preferences. You will hear music and sound fx as you trim dialogue, for example.
Martin, yes, I too often use the extend function. However, sometimes I am extending the length of a shot on the A side or B side, and the decision of the edit point is based on the sound (i.e., the dialogue). If I wish to use dynamic trimming and play the A side forward as I make the trim, I want to be able to hear that audio as I go (as opposed to hearing the audio of the B side as it currently sits in the timeline). If the video and audio of the given clip is not linked, I will not hear it unless I have selected “trim with selection.” (Which prevents me from hearing audio when I am doing a video-only dynamic roll trim.)
Hopefully Apple will address this issue.
Sherwood