Well, there is only slight difference - if it is plural you pronounce les - "le". If it was just one, it would go le garçon, le pronounced as "l". So you just have to listen really close, because if you miss the les/le pronounciation difference, you are lost :).

you're right, "garçons" starting with a consonant, you should hear @ "lay" not "layz" and garçons with no "ss" or "z" sound at the end.

No it couldn't, because "les garçons" is plural. If you don't hear the differences, check with the slower button.

Listening to the slow-mo is important. However, be aware that the recording is sounding out each individual word and so your aren't going to hear the liaisons, ie, "ils sont au jardin" will not have the "t" sound in "sont" pronounced, as it should have.

I.e that could as well mean "the boy eats good bread"...

When written it's clear. My question was if there is an actual difference when spoken. To me there wasn't :) :/ You wouldn't pronounce the s at the end of les because it's not followed by a vowel or am I totally lost here ?

Yes. I will give you an example. Take the word "grand"(big). If you talk about man: l' homme grand- you don't pronounce "d". But for female: La femme grande - you pronounce "d".

"es" in "les","mes","tes","ses","ces" is pronounced like a "è". So "Les" is pronounced exactly like "lait". (ait is also pronounced as an "è")