Apple Final Cut Express 4 - Manual (Page 536)

536 Part VII Fine-Tuning Your Edit
To hear only the selected audio tracks in the Timeline while using the J, K, and L keys
in the Trim Edit window:
1 Choose Final Cut Express > User Preferences, then click the Editing tab.
2 Select the Trim with Edit Selection Audio (Mute Others) checkbox.
Important: Make sure the Trim with Sequence Audio checkbox is also selected.
3 Select edit points in the Timeline and double-click them to open the Trim Edit window.
4 Move the pointer over the side of the Trim Edit window you want to listen to, then use
the J, K, and L keys to play back the incoming or outgoing clip.
Note: When you are working with multiple edit points in the Timeline, selecting an
audio track from the Track pop-up menu allows you to listen to only that audio track
during playback on either side of the Trim Edit window.