First go to 'Help' in the main menu (or F1 shortcut) and check the contents section 'audio' or 'adding sound' or such, the title is different with program versions. This should give you a general overview.
Briefly you will have to decide what, if any, of the audio that is linked to you video will be used, and add additional sound by placing audio clips eg .wav or .mp3 files onto the editing timeline. Just like video objects, audio can be cut and trimmed and overlapped to make fades between different section. It's really not possible to give a full tutorial here but do come back if you have more specific questions.
Thanks Carl for your reply, I probable wasnt to spacific, I need to trim a soundtrack [ I can do the full track, no problem] to about 30 seconds prior to some video coming on, then carry the sound after the video, I have no problem putting the music on , its just fine tuning information I need.
First, you can simply make cuts in the audio track ( [T] key shortcut ) and delete what's not needed. Note that editing in MEP is 'non-destructive' -- cuts on the timeline don't change your original file.
Second, do a right click on you audio track and pick 'Volume curve' from the menu. Here I've created points and moved the a section of the yellow line to nearly eliminate a distracting noise.