When the sound waves are not in sync, they cancel each out out. Similar to the way our active noise cancelling headphones works - but in a bad way.
Your left and right speakers may be wired "Out of Phase". The result is you will hear very bad sound. This leads one to think the speakers are defective when they are in fact not.
Make sure you connect the left and right satellite speakers using the same polarity. If a speaker wire goes into Left speaker's red jack, that same wire has to go into Right speaker's red jack.