Velocity envelopes are only for video.

However, if you want a constant slow speed for both video and audio together, trim (or split) your clip to just the section you want slowed down, then use the Ctrl-Drag method on the end of the clip to stretch it out. This will slow both the audio and video down together.

The default operation is to preserve the pitch of the audio. This sounds ok up to a certain point, but if you go farther can make it sound very "robotic". If you want the pitch to decrease as well, right-mouse-button click on the audio event, choose Properties, Audio Event, and set the Time Stretch/Pitch Shift method to "Change length and pitch".