Sound effects add a level of feedback to your game that, when missing, will be very noticeable to players. Even in situations where sound effects are not realistic (you would not actually hear explosions in space), their absence would feel wrong while playing.
There are a number of sound effects freely available on the web that can be found with a little searching. Locating truly royalty free sound effects is a bit harder. Most of the sound effects archives on the web are not the actual originators of the sound files, so their true licensing requirements may be difficult to track down.
You can always record your own effects if you have a way to simulate sounds. In the case of Asteroid Belt Assault, the sound effects were generated using a program called sfxr. This application, available at http://www.drpetter.se/project_sfxr.html includes full source code, and produces basic sound effects via random generation.