If these are typical linked audio/video clips, a CROSSFADE video transition will also crossfade the audio.
If both clips do not have enough excess unused content (that is not be seen on the timeline) to accommodate the fade duration, and the OPTIONS | EDITING tab Video transition default setting is Ask Me, a prompt will appear...
If freeze is chosen, audio on the FROM clip will fade out and audio from the TO clip will fade in. If overlap is the choice, audio will crossfade.
If the crossfade is to be applied to only the audio tracks, offset the two tracks. Position the red-lined cursor where the fade is to begin, right-click either audio track and choose CROSSFADE. Set a duration. The audio tracks will then look like this. Note the blue lines that show the fade...
