sound-juicer's docbook help files try to include legal.xml but do not install legal.xml.
This causes yelp-build html to exit non-zero.

    warning: failed to load external entity "/usr/share/help/ar/sound-juicer/legal.xml"
    /usr/share/help/ar/sound-juicer/./index.docbook:34: element include: XInclude error : could not load /usr/share/help/ar/sound-juicer/legal.xml, and no fallback was found

Here is a minimum recipe to reproduce the problem:

    mmdebstrap forky /dev/null --include=sound-juicer,yelp-tools --customize-hook='chroot $1 env --chdir=/usr/share/help/C/sound-juicer yelp-build html index.docbook'

Here is a minimum recipe that also does a couple of other packages, to show they work OK:

    mmdebstrap forky /dev/null --quiet --include=sound-juicer,yelp-tools,docbook-xml,aisleriot,four-in-a-row,kalgebra-common,kdoctools6 --customize-hook='chroot $1 find -O3 /usr/share/help/ /usr/share/doc/HTML/ "(" -name index.docbook -o -name "*.page" ")" -printf attempting: -print -execdir yelp-build html --path /usr/share/kf6/kdoctools/customization/dtd/ {} ";"'

These legal.xml files are present in the source tarball.
They should be installed by the install process.

I think this is the file to fix; you can compare with other gnome help packages:

https://sources.debian.org/src/sound-juicer/3.40.0-5/help/meson.build#L3
https://sources.debian.org/src/four-in-a-row/1%3A3.38.1-6/help/meson.build#L11  (using mallard .page)
https://sources.debian.org/src/aisleriot/1%3A3.22.35-6/help/meson.build#L79     (using index.docbook)