Oh, you are right, actually, I have forgot to mention that, indeed, in some Slavic languages, this vocal change is not observable in orthography.
I have completely overlooked that Slovene is actually, a language which belongs to the second group, since it has this phoneme, it just doesn’t distinguish it in writing.
This can also be observed in my language, Slovak. Here, “V” can sometimes change to “W” or “F”, but, as for the “L”, that is present in my speech, but, however, I believe that it isn’t a characteristic of literary Slovak, just my dialect. Here, we also pronounce:
“On je pil” as [on je piw]
and “zabával sa” as [zabávaw sa].