Alone or before a word starting with a consonant: no difference ideed, the "s" is mute. But you must consider the "liaisons" if the following word starts with a vowel or a mute "h". Add a "z" sound to both "ils" and "elles":
Ils aident = eel'z'ayd
Try "ils aident" (like above) and "il aide" here:
http://www.research.att.com/~ttsweb/tts/demo.php#top
and you'll hear the "z" sound (rather choose Juliette..French btw)
"Liaisons" are important and required in French. Read about it here:
http://french.about.com/library/pron...l-liaisons.htm
Examples:
http://french.about.com/library/pron...liaisons-r.htm
(sounds clickable)
But there are also forbidden "liaisons"!!
http://french.about.com/library/pron...liaisons-f.htm