I have a couple of Walkmans that seem to have the same problem - the sound at the start of most tapes is variable (volume goes up and down, muffled etc). This seems to only last for a couple of minutes though and then everything is fine. And it is only the very start of the tape.
I thought it might be that the actual tapes are old and worn, but the same thing happens with new tapes. One of the Walkmans is reasonably new too, so it can't be that.
Is it just that they don't make things as good as they used to??