How to remove horizontal line on audio track

Just reached out to customer service and this is how they had me solve the problem- click on the wrench on the left side of your timeline, and unselect "show video keyframes" and "show audio keyframes." Worked like a charm!

That line defaults to representing Volume > Levels.  This can be changed by right-clicking the FX badge for the audio clip and selecting something else.

The line shows when the Audio Track height is tall enough for it to be displayed.

There are several ways to reduce the Audio Track height so that the line will be hidden.  I like to use the Alt/Option + Minus keyboard shortcut.

The Customize timeline track controls Adobe Help video by Maxim Jago covers adjusting Track Height in the first segment of the video, but it has a lot of helpful information.  I recommend watching all of it.

That's the 'volume' or level indicator. That shows say if you have keyframes for your audio levels.