Hi there,
try this. If you look on the audio file, you will see a horizontal line which is either near or in the middle of the file. You should see small rectangles. including those to the left and the right.

Place the cursor, and keep left mouse button depressed, on the middel rectangle and you can drag the whole line upwards, (volume up) and downwards (volume down). Do the same with either the rectangle on extreme left or right, and you will have created a fade in/ and out as you need.

Cheers,

M o' L