Place the full video and the full Mp3 clips on their respective tracks, then sync them, as instructed earlier, by moving the track that's early to the right so it matches up with the other track. Once done, select all tracks, right-click on one of them and GROUP SELECTED CLIPS. This will lock them together. Now is the time to SAVE PROJECT AS, so you can always restore.
You can then split all tracks where you want to begin, and at the spot where you want to end. Now delete the extraneous content with a right-click | DELETE (ripple) in the areas to be removed. If there are gaps, drag the content. Everything will move in concert because the tracks are grouped. Sync won't be changed. SAVE PROJECT AS once again, with a unique # or name. Anytime you make a mistake, press <ctrl-Z> (EDIT | UNDO) to go back step-by-step.
To monitor the mix while tweaking individual audio tracks for EQ, Echo, etc adjustments, click its track, then click the AUDIO EFFECTS button on the Home menu toolbar.
Isolate a track by clicking its SOLO icon 
at the left of the track. The volume sliders can also be adjusted, for relative gain.
To see and manage what effects are being applied to a specific track, click the FX icon at the lower-left of the audio clip. Click the X there to delete an effect.
If an effects window for a specific track is open you can click on the header to see its waveform in the preview window. Or double-click the track in the sequence. Drag the line up or down to set keyframes and plot a volume graph. Right-click on a keyframe dot to delete it. You will hear only the track whose waveform is being displayed, unfortunately. The ability to hear the mix in this mode is on the wish list.