I've never heard of or used the software you are using but generally you need to zoom into the audio track and find the part where you want the fade to start and set a keyframe (a marker where an event occours - such as you wanting to fade the audio out) and another one at the end of the audio where it is gone completely. The first keyframe will have the audio set at 100 per cent and as you go towards the end of the audio track it will fade down to 0 per cent. You might also try an audio effect or transition that you can just drop onto the end of the audio track and get the software to do all the work for you instead, as I say I have never used this software.

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