Fade audio linearly?

When you create the fade just right-click on the handle and select "linear".

Ah, I thought you meant a fade out. I don't know if you can change the curve to linear in the middle of a clip when using keyframes. You could use a workaround, depending how many you have to do: use the blade tool at the point you want the fade; double click the incoming track so it drops down and extend it under the outgoing; do a linear fade on the outgoing and adjust the incoming level to where you want the volume to end up. That'll give you the faster drop of a linear.

If you go into the Effects browser (Effects) and go below the video effects, there are a lot of Audio effects. Go down to Levels, and in the Logic section there is an effect called Gain. Apply it to the clips you need to animate.

Select the clip(s) you added Gain to and type Control-A (show audio animation). You should see something like:

Where you see Gain: All - click the white disclosure triangle and select Gain (dB) from the dropdown.

Then, click on the disclosure triangle at the right edge of the Gain animation (or double-click in the keyframe area); you'll get an editable pane like:

You can keyframe "linear" fades in the same way you would from the "plain" audio track - Option click on the line to create keyframes and drag to the position you require.

Linear is in quotes because it's just the change in gain, like a fader. Audio in itself is not linear, it's logarithmic. The audible effect of a linear change in levels is a rapid (exponential) falloff. The normal keyframe behavior in FCPX is correct for an audible gradual "linear-sounding" fade out or in between levels.