/usr/src/linux-headers-4.4.0-87-generic/sound is in linux-headers-4.4.0-87-generic 4.4.0-87.110~14.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.4.0-87-generic/../linux-headers-4.4.0-87/sound