Okay, i found the answer in the FAQ at the SoX homepage
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Question:
Why can't this wav file (with more than 2 channels or bit-depth more than 16) that I created with SoX be read by other programs?
Answer:
Unfortunately, there are 2 variants of the WAV-file standard for such files: an unofficial one, and the official one from Microsoft. SoX can read such files regardless of which standard they conform to, but some applications can cope with only one of the two variants. By default, SoX creates such files according to the official standard; to create such a wav file that conforms to the unofficial standard, `-t wavpcm' must be given before the name of the output file.
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So adding the option "-t wavpcm" to my small script solved the problem. Obviously DSoundtool wants what they call "unofficial standard". Anyway, converting a whole directory and creating the DSND file is now done in less than 3 minutes

Regards, Manfred