The apparently missing file libfftreal.so.1 is in the same directory
as
the spectrum executable, but dpkg-shlibdeps does not seem to know to
look for it there.
For example in
debian/qtmultimedia5-examples/usr/lib/x86_64-kfreebsd-gnu/qt5/examples/multimedia/spectrum:
$ LD_LIBRARY_PATH=. ldd spectrum
libfftreal.so.1 => ./libfftreal.so.1 (0x0000000801242000)
$ file libfftreal.so.1
libfftreal.so.1: symbolic link to `libfftreal.so.1.0.0'
$ file libfftreal.so.1.0.0
libfftreal.so.1.0.0: ELF 64-bit LSB shared object, x86-64, version 1
(FreeBSD), dynamically linked,
BuildID[sha1]=dbbcf445566c4944a39504c3c2720654d813a305, stripped