You have the option to have YInformer play a sound if a target comes back as online.  On the Main Options tab at the top of YInformer, you can click the Play Sound button (circled in red above) and then select a sound file from the dropdown box below that.  You can also preview the sound by clicking on the little sound icon button to the right of the dropdown box.  This sound will play if anyone comes back as online for either the manual or continous scans.

Extra:

The sound files are stored in the YInformer program files and should not be moved or deleted or modified as it can cause program instability or crashing.