This is where the field "Copyright" of the soundfont is useful. Unfortunately only a few of them mention it and we are usually not able to find where the samples come from.
Thanks. Coming from Wikipedia, where practically ANY media has to have full attribution, I'm rather cautious.
So we need to trust the authors of the soundfonts when they are sharing a soundfont: the license of the soundfont should be applied regarding the more restrictive license of the samples comprised in it if they are not using their own samples.
That is something I can fully agree with.
I'm based in the United Kingdom, and the recent shift towards significantly tighter copyright laws here and in Europe (the recent EU Parliament vote being forefront) was what prompted the original question.