Actually, I think that 'jablko' can be pronounced with or without barred L and both pronounciations are regarded as correct. I read somewhere though that the one with barred L is sometimes thought to be hypercorrect or a ruthenism (which is an influence of east slavic languages).
There are also ther cases where barred L may (or may not) be silent, like 'padl' (he fell).
I cannot think of any other polish letters right now that may become silent.
Thanks once again for your help and, it's much appreciated.
No problem. :)
Vincent, you may find this website very useful.
It has a speech synthesizer, which means that it pronounces words that are typed in the window called 'Wpisz tekst do odczytania'. Just tick Jacek if you want to hear a polish man or Ewa if you want to hear a polish woman and press 'Odczytaj!'.