Thanks, guys, for the replies. I am afraid that I don't fully understand the remarks of Mag about the 2.1 soundtrack of a DVD. Anyway, when I changed the HDMI output of my Blu ray player from LPCM to Bitstream the sound was noticeably better; not completely free from distortion, but it was listenable.
I know that the SP3 has a good reputation but so has my Blu ray player: it is a Primare BD32. The transport is from Oppo, all the electronics are developed by Primare.
This afternoon I visited my dealer, to check the DVD and the sound was quite clean, no distortion. The Blu ray player had two options for HDMI out: Auto and PCM. I have tried them both but I couldn't hear a difference between the two.
Back home I checked another DVD of mine. The sound quality was decidedly better, no distortion at all. So my Primare / Bryston combo is capable of good results. Afterwards, I found out that the second DVD had a DTS soundtrack, but I am not sure if that matters.
So it is still a mystery to me what the problem is, but there is something wrong in the audio chain. Only what?