A lot of packages bundle their dependencies. Examples: rsync (contains a bundled libpopt), gnupg (contains a bundled libz). Other dependencies commonly bundled are gettext or glib. For inspiration look at how these popular open source projects do it.
Put the content of http://www.mega-nerd.com/libsndfile/files/libsndfile-1.0.25.tar.gz into a subdir and add apropriate rules to build the the subdir first.
Untested sample code: OBJS += libsndfile/libsndfile.a
libsndfile/libsndfile.a:
cd libsdndfile && ./configure --enable-static && $(MAKE)
For Bonus points add a configure script, that check if the system has already installed libsndfile and link to it dynamically.