You can right-click the audio track to add fades.
Right-click, then click CURRENT EFFECT PROPERTIES to see the audio clip. Create keyframes along the blue line by left-clicking and dragging.
Note also in recent versions, the added balance and level controls to the left of the audio tracks. (To make these controls visible use the far left icon to expand the display.)
It appears that the line in the timeline itself can no longer be manipulated. That, in my opinion, is a step backwards - despite the enhancements and CURRENT EFFECTS PROPERTIES alternative.
There is also a MIX icon available in the PROPERTIES box...
When editing effects for a track clip, you can choose to have the preview player mix with other clips as it would sound on the sequence. To enable or disable this mixing mode, click the Mix button.