Could also be an audio codec used that isn't licenced by Mutant.
Is that the stream in the "Stream NL nationaal (NL)" bouquet, that is marked "++NPO - werkt niet in iedere ontvanger!++" ?
According to VLC it has A52/AC3 audio, which should not be a problem for the HD51, if you have an AC3 capable audio system (or TV), of if you have AC3 downmix active.
Yes, that stream. But video is working fine.
Downmix or passthrough don't make a difference. I do have a AC3 capable audio system and TV. There is no problem with the AC3 audio when receiving NPO 1 HD by satellite.