Para instalar um pacote de Backports você precisa editar o seu sources.list
:
echo 'deb http://http.debian.net/debian jessie-backports main' > /etc/apt/sources.list.d/jessie-backports.list
Em seguida, instale o pacote necessário:
apt-get update
apt-get -t jessie-backports install libswresample-dev
NB
Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care!