pretty sure it's in the settings, transitions between clips you want set to fade and and that will fade sound and video - but I think what you want is hard cut the visuals and fade audio (maybe a nice auto audio request that could be just in replays) which im not sure if theres a work around yet
(Martin, as an videomaker/editor - you almost never use crossfades, one of our rules is to not use them for editing anymore, but you would always always need to fade audio)

but I understand the auto audio fade is a can of worms that really needs sussing out