There are a few subtle sound differences - like everyone's been saying, le, la and les all sound different. If you hear that 'ay' sound from les or des, you know it's plural.
Sometimes you also get to hear the 's/z' sound when it's followed by a vowel:
les enfants = layzonfon
l'enfant = lonfon
Other times you get a cue from the verb:
Elle lit le livre = elleeluhleevr
Elles lisent le livre = elleezluhleevr
(I just started with French so those might be pretty bad phonetic transcriptions, but hopefully you get the idea!)
My advice is to avoid looking at the screen, keep pressing ctrl+space (or whatever shortcut you have) to play the audio, and listen carefully to the article (le/la/les) and the verb and see if you can pick out the defining sound. Eventually it'll become more like second nature, and your brain will just pop the relevant idea of number and person into your head, like with your first language. English isn't a whole lot different, we differentiate with a couple of s/z sounds too!
I realize that I'm not SiteSurf but wanted to point out that there is a difference between the two.
"Le" is pronounced "luh" and "les" is pronounced "lay" (or alternately it uses the e-sound used in "beer").
That's the only thing that sets the pronunciations apart.
I'll let SiteSurf add to that if he/she wants to.