One property of life is communication. In particular sound is very useful due to it's non-directional nature. An alarm call is heard by every member of the pack and warns them immediately of the danger whether they are looking in the right direction, or asleep, or doing something else entirely. It also travels around corners, allowing you to hear the alarm call even if you have something in between you and the lookout.
For life-forms evolved in space, for example Could life form in outer space? then sound would be almost completely ineffective. It could be used when two life-forms are touching each other or touching the same solid body but not when they are separated.
What could they use to serve the same purpose as sound does for earthly lifeforms? A mechanism that they could use for communication with each other and that has similar useful properties to sound waves. Is there anything that could take the place of sound as an alarm, warning, or distress signal?